Though there were no tears, quiet grief was etched on the faces of many of the 300 or so people who gathered near the now derelict Union Carbide plant in this central Indian city. Though most were sad, a few were angry.

After lighting their candles they arranged them to make the figures eight, four, zero, four.

"This arrangement of candles signifies the passage of time from 1984 to 2004, marking the 20th anniversary of the tragedy," said activist Satinath Sarangi, who heads the International Campaign for Justice in Bhopal.

Men and women, many of them elderly, lined up with children behind the flaming candles at a memorial -- a stone statue depicting a woman with her children -- clutching fading black and white pictures of loved ones killed on that night of horror 20 years ago.

More than 3,500 people died during and immediately after the accident when some 40 tonnes of lethal methyl isocyanate escaped from the Union Carbide plant at around 12.30 am on December 3, 1984.

Indian government records say the total death toll from the tragedy in the past two decades is about 15,000 but local victims' rights activists say the figure is double that.

Amnesty International in a report Monday said 22,000 to 25,000 people had died after the accident.

Musharraf Ali, 51, who lost his wife Afroze to blood cancer in 1996, was among those grieving in the quiet of night near the factory.

"Doctors told my wife and myself that she had blood cancer in 1993 and within three years she was gone," Ali said.

"I come here for the vigil every year to express my sorrow at the horrific accident that destroyed my life."

A number of activists from India and overseas, including French author Dominic Lapierre, were also present at the vigil that began around 10.00 pm (1630 GMT) and was to continue into the early hours of Friday morning.

Most of the gathering had earlier in the evening assembled with burning flame torches in Bhopal's main Shajahani Park.

After lighting the torches, the victims -- mostly women widowed by the gas leak -- marched for about a kilometre (mile) towards the factory shouting slogans such as "Death to Union Carbide!"and "Union Carbide, quit India!"

Seventeen-year-old Tahir Khan said his mother had sent him to the vigil as a tribute to his father, who died from the after-effects of inhaling the toxic fumes.

Earlier in the day, participants of a seminar on the issue, demanded that Dow Chemicals, which took over Union Carbide in 2001, pay the medical expenses of the 800,000 or so people injured in the gas leak.

They also urged Dow to clean up the site that activists say contains thousands of tonnes of toxic chemicals.

The demand was voiced just after an Indian official said the first steps had been taken to finally remove waste from the site.

"The federal government has asked (state-run) Engineers India Limited to survey the Union Carbide plant to determine how much toxic waste there is on the site and how it can be disposed of," said Uma Shankar Gupta, a local minister in charge of relief for victims of the tragedy.

"The survey report of the Engineers India Limited will be the first step towards the cleanup of the plant," Gupta told AFP.

But global rights group Greenpeace said the steps were not enough.

"While we welcome the announcement that the Indian government wants to take proactive and urgent measures to conduct the clean-up, Greenpeace believes that after 20 years this is the slowest first step in history," said Vinuta Gopal, Greenpeace India campaigner.

"Greenpeace calls on the Indian government to immediately commit itself to most urgent and immediate measures of securing the site and providing clean drinking water to the survivors," Gopal said.

Fresh graffiti and skull and crossbones signs, meanwhile, were painted by activists on the walls surrounding the Union Carbide plant warning people to keep away.

One picture showed a group of women beating former Union Carbide chief Warren Anderson with broomsticks, while another showed parents rushing children to hospital.

Anderson, who has a charge of culpable homicide outstanding against him in India, and his company have been accused by human rights groups of cutting corners on safety features to save money.

On Friday local government representatives were to pay their respects at an official function in the city, while victims and activists were planning to hold a rally to mark the day, an organiser said.

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