The 15- to 20-foot (4.5- to six-metre) long mammal swam upstream through the heart of the British capital past landmarks like Big Ben and the London Eye ferris wheel as amazed onlookers crowded the riverbanks.

Britain's Saturday newspapers ran the story and accompanying photograph on their front pages. "Save The Whale," The Independent said.

A second whale, believed to be of the same species, was spotted at Southend at the mouth of the Thames estuary, said the British Divers Marine Life Rescue (BDMLR) charity.

The rare whale, which normally lives in deep water, became briefly stranded in the shallows around Chelsea in west London and people waded into the river to try to encourage it back into the channel.

Fears grew that the whale might become stranded again when it headed back upstream as dusk fell, with the tide set to go out again later in the evening.

The whale was followed by a flock of gulls, a helicopter beaming blanket television coverage and boats making sure it did not tangle with any river traffic. Onlookers cheered when it surfaced to spout water from its blowhole.

The Royal National Lifeboat Institution (RNLI) told AFP the whale was tiring and police said they thought it might be injured.

The BDMLR's Tony Woodley, monitoring the whale's movements, told AFP the group's members were at the ready if the mammal beached again.

However, they would not intervene as long as the animal was swimming freely.

He said the second whale off the Southend coast could be an adult while the whale in the Thames was a youngster.

"This animal has stranded twice. This is very concerning," he said.

"It might be unwell. It's certainly very stressed and in relatively shallow waters. These are deep-water species, not coastal or estuary ones.

"For an animal such as this to be where it is is very unusual and concerning."

Woodley said that an experienced marine mammal vet was on his way to the scene, as was specialist pontoon equipment to possibly refloat the whale should it beach.

Richard Sabin, an expert from the Natural History Museum, said it was the first sighting of a northern bottle-nosed whale on the Thames since records began in 1913.

The whales, normally found in deep offshore waters in the Arctic Ocean and the northern Atlantic, can grow to 10 metres (33 feet) long and weigh up to eight tonnes.

The population has been severely depleted due to commercial whaling.

A spokeswoman for London's Metropolitan Police said officers had been alerted just after 9:00 am (0900 GMT) by members of the public watching the whale at Jubilee Bridge over the Thames.

"The whale is not believed to be in a good condition and may be injured. Police are working with the RNLI to ensure the safe navigation of craft on the river while the whale is in the Thames," she said.

The RNLI sent a boat out to monitor the whale's health.

A spokeswoman said: "It was found to be very tired. They won't intervene unless it becomes stranded and a further assessment will be made."

The BDMLR said late Friday the whale had not been spotted since it was last seen near Albert Bridge at about 5:15 pm (1715 GMT).

Spokeswoman Alison Day said: "We were hoping that when the tide changed, the whale might reappear and we would be able to assess its condition. That has not happened.

"What we believe may have happened is that the whale has become quite distressed by all the noise and boats on the river, and it has dived deeper into the water.

But Day said the BDMLR was keeping a boat on the river until it finds the whale again.

Dolphins, seals and porpoises have all been unexpected visitors to the Thames in recent years, a testament to the recovering state of the once biologically-dead tidal river.

Over the last 30 years, the Thames in London has become one of the world's most unpolluted metropolitan rivers, with a variety of wildlife flourishing again in its waters and along the banks.

The Zoological Society of London recorded the sighting of 103 marine mammals in the Thames in the year to June 2005, mostly near the estuary. They included more than 100 seals, 62 porpoises and 18 dolphins.

Sperm whales -- which grow up to 46 feet (14 metres) -- have been spotted in the Thames estuary. Ten years ago a minke whale died after becoming stranded on the shore of the Thames at Purfleet, downstream from London.

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