There was no immediate word on the full extent of casualties and damage, but at least two people were reported killed and dozens injured in the quake, which split open buildings 300 kilometres (185 miles) from the epicentre.

In the capital Jakarta 600 kilometres further south, high-rise towers wobbled, water sloshed from swimming pools and panicked office workers ran into the streets. Elsewhere, power was knocked out and phone lines went dead.

The huge quake -- anything over magnitude 7.0 is considered to have the possibility for widespread damage and loss of life -- was felt in neighbouring Singapore, Malaysia and Thailand, where office buildings swayed and shook.

The US Pacific Tsunami Warning Centre said an alert was raised for the entire Indian Ocean area including Indonesia, India, Sri Lanka, Thailand and the Maldives -- all affected by the devastating December 2004 Asian tsunami.

"Sea level readings indicate a tsunami was generated. It may have been destructive along coasts near the earthquake epicentre," the Hawaii-based centre said.

"For those areas, when no major waves have occurred for at least two hours after the estimated arrival time or damaging waves have not occurred for at least two hours, then local authorities can assume the threat is passed."

Suhardjono, head of the earthquake division of Indonesia's meteorological agency, said tidal gauges off the Sumatran city of Padang had recorded an ocean surge of no more than half a metre (a foot and a half).

"There was definitely no damage caused by waves anywhere along the west coast of Sumatra," he told AFP.

The undersea quake erupted around 1100 GMT some 100 kilometres southwest of the city of Bengkulu at a depth of roughly 30 kilometres, the United States Geological Survey said.

It adjusted an earlier report of magnitude 7.9 to 8.4.

Indonesia's meteorology agency said several aftershocks were recorded after the quake, which hit on the eve of the start of the Muslim Ramadan holy month, including one at magnitude 6.6 that prompted a second local tsunami alert.

The deputy chief official from North Bengkulu district told ElShinta radio that one person there had been killed by a falling tree while trying to evacuate, and that some buildings were "severely damaged."

The state-run Antara news agency reported that a 25-year-old woman had been found dead in her collapsed house in Bengkulu city.

Her sister told the agency that the woman had been bathing her four-year-old son when the quake hit. The boy's condition was unknown.

Rustam Pakaya from the ministry of health's crisis centre confirmed the toll was two and at least 11 people were injured. He could not confirm a report citing a Social Affairs ministry official saying seven were dead.

Dozens of people were injured in damaged buildings, said the district official, Salamun Haris. The Bengkulu airport terminal was cracked but the runway was in good condition, another official said.

There were several reports that the damage did not at first seem severe.

"I saw some parts of houses crumbled to the ground but not huge damage. People ran out of their homes," Bengkulu resident Ayu Claudia said in a brief conversation before the phone lines went down.

Budi Darmawan, a policeman in the Indonesian town of Mukomuko on the west coast of Sumatra, said buildings three storeys and higher had collapsed and that tsunami warning sirens had failed to activate.

"Buildings of three floors or more are either fissured or collapsed," he told ElShinta.

He said police raced through the streets on motorcycles, warning residents to move quickly to higher ground.

Malaysia, Sri Lanka, the Maldives, India and Mauritius all issued separate tsunami warnings telling residents to move away from the Indian Ocean coastline. Sri Lanka, Malaysia and India later lifted their warnings.

Hundreds of thousands of people in southern Bangladesh fled their homes in panic after the government issued a tsunami alert, while East African countries Kenya and Tanzania also urged residents to stay away from coastal areas.

Indonesia has endured repeated major quakes in recent years, including the 2004 quake that unleashed a tsunami across the Indian Ocean. It killed over 220,000 people in a dozen countries including some 168,000 in the Indonesian province of Aceh alone.

In May 2006, a quake rattled the country's main island of Java, killing more than 5,700 people and destroying some 300,000 homes. Two months later, another quake on Java killed more than 600.

In March yet another large quake hit Sumatra, killing more than 70 people, flattening buildings and displacing more than 1,700 people.

Indonesia, an archipelago of some 17,000 islands, sits on the so-called Pacific Ring of Fire, where continental plates meet -- and where earthquakes are a frequent and often deadly occurrence.

Massive aftershocks shake Indonesia
Huge aftershocks rumbled across Indonesia's Sumatra island on Thursday, as rescue teams sped to the area to assess the destruction the morning after a massive 8.4-magnitude earthquake.

The follow-on shocks, including one as strong as magnitude 7.8, drew new tsunami warnings from Indonesian authorities and panicked thousands of people who had already spent the night outdoors afraid of being buried alive.

At least six people were reported dead and dozens injured in Wednesday night's huge quake, and officials said they feared the death toll could still rise. In many places, phone lines and electricity were down.

The quake was strong enough to shake buildings in Thailand and Malaysia, and set off a tsunami alert as far away as eastern Africa -- raising memories of the December 2004 tsunami catastrophe that killed 220,000 people.

But the initial reports from some parts of the Sumatran coast suggested that this nation of 17,000 islands, victim of some of the world's deadliest earthquakes in the past, may have been spared the worst.

The healthy ministry's crisis centre said six people had been confirmed dead and at least 38 had been injured in the quake, which hit underwater some 300 kilometres (185 miles) off the Sumatran town of Bengkulu.

"My wife was washing the dishes and my two sons were taking a bath," said Samsul Anwar, sitting in a tent at the main hospital in the town. He had rushed home when he felt the ground shake violently.

"The roof of the house had collapsed, hitting my wife in the head," Anwar said, comforting his son lying injured on a stretcher. "She is dead."

Fearing any further damage, hospital authorities have moved intensive care patients and others to outdoor tents.

The powerful quake, which came on the eve of the Muslim holy month of Ramadan, toppled buildings and split open others in at least one town, Mukomuko, on Sumatra's western coast, local police said.

On Wednesday, the US Geological Survey gradually upgraded the strength of the quake from 7.9 to 8.2 to 8.4. Any quake over 7.0 is considered strong enough to cause massive destruction and heavy loss of life.

The quake was powerful enough to slosh the water out of swimming pools in Jakarta, hundreds of kilometres away, and scare office workers into racing out of their high-rise towers in Malaysia.

In Bangladesh, hundreds of thousands of people along the coast went running for higher ground after the government issued a tsunami warning on Wednesday night.

"Around half-a-million have left their homes," said Ashrad Shamim, a top official from the district of Chittagong city. "There's a panic."

Tsunami alerts were also issued Wednesday night in India, Malaysia and Sri Lanka. Indonesian authorities briefly issued two new alerts on Thursday morning, and Australia issued one for its Indian Ocean territories.

Indonesia lost 168,000 people in the province of Aceh alone in the 2004 Indian Ocean tsunami disaster.

The country sits on the volatile "Ring of Fire", a massive zone of volcanic instability that encircles the Pacific.

related report
Six dead, dozens injured in Indonesian quakes
At least six people have been killed in powerful quakes off Indonesia's Sumatra island, but the toll is likely to rise amid poor communications to affected areas, officials said Thursday.

"Five died in Bengkulu and one in Padang," Rustam Pakaya, head of the health ministry's crisis centre, told AFP, adding that at least 38 were injured and being treated at hospitals in the two coastal towns.

The hardest hit area by an 8.4-magnitude quake that struck on Wednesday and has been followed by a series of major aftershocks appeared to be North Bengkulu district, he said.

He said a wave 1.5 metres (yards) high swept into Sikakap beach, on the island of South Pagai off Sumatra, reaching houses "but so far we have no reports of human casualties from the incident."

Health ministry spokeswoman Lilies Sulistyawati said the toll was expected to rise "as communications with Bengkulu have been difficult."

She said a team of surgeons were set to leave for Bengkulu and ambulances from Palembang had been dispatched to the coastal city as well.

Sumatra's west coast was hit by a second 7.7-magnitude quake on Thursday morning, panicking already frightened survivors. related report
Tsunami panic hits southern Bangladesh
Hundreds of thousands of people in southern Bangladesh fled their homes in panic fearing a tsunami after a major earthquake off Indonesia, officials said on Wednesday.

Local officials said some 600,000 people rushed from coastal regions of the disaster-prone country following a government tsunami warning.

Police with loud-hailers raised the alarm after the 8.4-magnitude earthquake hundreds of miles (kilometres) south in the Indian Ocean.

"Around half-a-million have left their homes. They've taken shelter in schools, colleges, cyclone shelters and relatives' houses," said Chittagong district administrator Ashraf Shamim.

"There's a panic but we're using loudspeakers to ask people to take shelter in safe places."

An urgent government warning that a tsunami could hit after midnight was repeated frequently by both state and private television and radio stations. It was finally cancelled at 1:30 am Thursday (1930 GMT).

Indonesia, Malaysia, Sri Lanka and India also issued tsunami alerts but cancelled them hours earlier as the threat of giant waves receded.

"We started using loud-hailers at 8:00 pm (1400 GMT) after the government's order," said Mahbubur Rahman, police chief of the southern island of Sandweep.

"So far some 70,000 people have been evacuated to cyclone shelters, colleges, schools and government administrative buildings.

"They have left their homes and are huddled together at the centers."

The US Pacific Tsunami Warning Centre issued an alert for the entire Indian Ocean area including Indonesia, India, Sri Lanka, Thailand and the Maldives -- all affected by the devastating December 2004 Asian tsunami.

But the centre said later that the danger had passed.

Bangladesh, a frequent victim of flooding and ferry disasters, escaped the 2004 Indian Ocean tsunami which killed 220,000 people in a dozen countries after another massive earthquake off Indonesia's Sumatra island.

But officials, unwilling to take chances, opened disaster control rooms in the capital and the districts to coordinate the evacuation after Wednesday's quake.

"The district administrations in the coastal areas have been ordered to open temporary shelters so that people can stay the night there," said government press spokesman Mahbub Kabir.

Tens of thousands were ordered to take shelter in the southern district of Cox's Bazar, while ships were ordered to stay close to harbour in Chittagong, home of the country's largest port.

"It's massive work. But we are going to take all the people to safe places," said Chittagong official Shamim.

related report
East African nations issue tsunami alerts
African nations on the Indian Ocean on Wednesday warned of a possible tsunami after an earthquake struck off the west coast of the Indonesian island of Sumatra, officials said.

The Kenyan authorities advised residents along the coastal region to keep off the beaches and remain alert, warning that a tsunami was expected.

Government spokesman Alfred Mutua, who hard earlier said the "a massive tsunami is expected to hit the Kenyan coast at 11:39 pm (2039 GMT)," downgraded the alert.

Mutua said people should not evacuate the beaches -- including hotels -- as earlier called for, but instead be watchful.

Neighbouring Tanzania's meteorological agency also issued a warning urging residents living along the coastline and on the islands of Zanzibar and Pemba to be vigilant.

"The earthquake has generated a tsunami. It is expected that the tsunami waves will hit Tanzania coastal areas as from 11:30 pm (2030 GMT) tonight," the agency said in a statement.

The government of Mauritius issued a tsunami alert that was broadcast on radio stations, urging the tiny Indian Ocean island's inhabitants to keep off beaches at night.

A response committee was formed and residents in some of the most exposed coastal regions were encouraged to gather in designated areas for the night.

Earlier, a massive 8.2-magnitude earthquake struck off Sumatra island, toppling buildings. It triggered an Indonesian tsunami alert, which was rapidly called off.

Indonesia has endured repeated major quakes in recent years, including the 2004 quake that unleashed a tsunami across the Indian Ocean.

It killed over 220,000 people in a dozen countries including some 168,000 in the Indonesian province of Aceh alone.

That tsunami killed around 300 people in Somalia, caused damage in Africa's Indian Ocean islands of the Seychelles and Mauritius. It also hit the coasts of Kenya and Tanzania.