Amazed onlookers crowded the Thames riverbanks Friday as the 15- to 20-foot (4.5- to six-metre) long mammal swam upstream through the heart of the British capital past the Houses of Parliament and the London Eye ferris wheel.

Saturday newspapers ran the story and photographs on their front pages. "Save the Whale" was the choice of words from the environmentally-minded Independent. "Free Willy!" the mass-circulation Daily Mail appealed.

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. It disappeared for hours until it was sighted again before 1 am (0100 GMT) far upstream by Battersea.

Tony Woodley, of the British Divers Marine Life Rescue (BDMLR) group, which is on standby to rescue the animal if it beaches, said several medics were at the site with rescue equipment.

Medic Jamie Henn said the whale stranded briefly before they could reach it and it then freed itself from the bank.

The Port Authority launch confirmed the river is only 2.5 meters (eight feet) deep there.

On Friday 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 Woodley, monitoring the whale's movements, told AFP earlier that the group's members were at the ready if the mammal beached again, but would not intervene as long as the animal was swimming freely.

He said a second whale spotted 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."

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."

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.

All rights reserved. � 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.