The lost and distressed whale stranded in the River Thames died Saturday as rescue workers ferried it on a rusting salvage barge in an effort to release it in the open sea, an animal rights group said.

The 20-foot-long Northern bottlenose whale had been lifted onto a barge by rescuers and was being taken downriver toward the North Sea when it suffered convulsions and died, the Royal Society for the Prevention of Cruelty to Animals said.

The whale struggled with the effects of being out of the water as it was ferried toward the Thames Estuary, officials said.

"It was a brave, valiant, but ultimately tragic effort to get the whale to safety," RSPCA scientific officer Leila Sadler said.

Swaddled in blankets on the barge, the marine mammal — watched by thousands in London as it spent two days swimming up the murky river past some of the capital's most famous landmarks — had shown signs of increasing stress and stiffening muscles, an indicator it was in serious difficulty.

"The animal suffered a series of convulsions at around 7 p.m. (2 p.m. EST) and died," Sadler said. "It was already dehydrated, hadn't been feeding and the being out of the water would have, in effect, shriveled the animal's internal organs.

"It was essential to try to take the whale out to sea on the barge — but there was always the risk this would happen."

A crowd of 3,000 people at Albert Bridge in south London had cheered and applauded as the whale was tethered to a sling and lifted by a crane onto the barge Crossness. Rescue crews were heading toward Margate, on the southern English coast, where they hoped to let the whale back out to sea.

"There was a real chance that the rescue attempt could have succeeded, but these type of mammals are very prone to the effects of stress and I'm afraid it all became too much," said Tony Woodley, spokesman for the British Divers Marine Life Rescue group, which led the rescue attempt.

"It was always going to be a race against time to get it to the ocean, especially with the effect being out of the water has on a whale's body."

A veterinarian will conduct a necropsy aboard the salvage vessel to determine the cause of death.

"All the crew on the barge are shattered by the death," Woodley said. "They were tired and exhausted but had been determined to do everything they could to get the whale to safety. It really is a terrible shame."

Experts had warned earlier that the Northern bottlenose whale, normally found in the cold North Atlantic, may not survive. Witnesses said the mammal's snout was bloodied, and photos appeared to show damage to one of its eyes and a number of cuts on its torso.

Earlier, veterinarians and rescuers waded into the river near Albert Bridge to assist the whale, taking medical tests and attaching an inflatable pontoon to the animal as Londoners jammed the riverbanks to watch the drama. Blood samples were being tested at a nearby hospital.

The Northern bottlenose whale — the first seen in the river since record keeping started in 1913 — flailed through the Thames on Friday, passing Big Ben and the Houses of Parliament as hundreds of curious onlookers jammed the shoreline. The animal twice tried to beach itself.

International and local television networks broadcast the drama live, and police were forced to close a nearby bridge as the number of people watching swelled.

The Northern bottlenose whale can reach nearly 30 feet in length — longer than a traditional red double-decker London bus — and weigh nearly 8 tons. The whale was about 40 miles from the mouth of the Thames on the North Sea.

The whales are known as curious animals, readily approaching boats and normally traveling in groups, according to the Whale and Dolphin Conservation Society's Web site.

When sick, old or injured, whales often get disoriented and swim away from their pod, said Mark Simmonds, the society's science director.

Some people reported seeing a second whale in a different section of the river Friday.

Last week, marine officials said they saw two bottlenose whales in northeastern Scotland. The mammals are normally seen in northwestern Scotland. That, coupled with the second sighting Friday, could suggest that something is disrupting the whales, Sadler said.

Scientists have said fluctuating ocean temperatures, predators, lack of food and even sonar from ships can send whales into waters that are dangerous for the mammals.

"It's extremely rare for one to turn up in a river in the United Kingdom," said Tony Martin, a senior scientist with the British Antarctic Survey.