Hundreds gathered at Richmond Lock and Weir in south-west London on Sunday after the whale, a Minke between three to four metres long, became stuck on the lock’s boat rollers.
Videos showed it being hosed down by a man believed to be a Port of London Authority (PLA) staffer while a vet performed a check-up at the river’s edge, before the Royal National Lifeboat Institute (RNLI) arrived at the scene to the cheers of onlookers at about 9pm.
Glen Nicolaides, from London Fire Brigade, told the BBC the whale had been moved to a more “stable” location where it would be assessed to determine the scale of its injuries and whether it could be released.
But the whale was spotted near Teddington Lock at just after 10.20am, heading downstream towards Chiswick and back towards Richmond Lock and Weir.
A spokeswoman for the RNLI said: “Chiswick’s RNLI crew were tasked by London Coastguard at 8pm on Sunday May 9 and worked with the fire rescue service, police and British Divers Marine Life Rescue to try to free the whale.
“Together they managed to free it and were taking it to a deeper part of the river when it swam away.
“The whale was displaying concerning behaviours and experts in attendance were concerned that it may be unwell.”
London Fire Brigade station commander Glen Nicolaides, who was at the scene, said: “This was a very rare and unusual incident. Crews assisted the Coastguard, RNLI and other marine agencies at the scene.
“Our crews were on stand-by while a marine biologist and vet assessed the condition of the whale. The mammal was removed for further assessment by marine specialists.”
A witness told the PA news agency that “quite the crowd” watched as the attempted rescue took place on Sunday.
Jake Manketo, 20, from Richmond, said: “Everyone here is just hoping they get it out.
“We couldn’t believe our eyes when we first saw the poor fella, not every day something like this happens in Richmond.”
It is believed the whale was first spotted at midday a few miles up the river near Barnes Bridge.
A spokesman for the PLA, which owns and operates the lock, said: “At around 7pm on Sunday, a small whale, approximately 3-4m long, believed to be a Minke whale became stranded at Richmond Lock and Weir.
“PLA staff have attempted to assist the whale with water along with British Divers Marine Life Rescue.”
Minke whales are the smallest of the great whales, growing to about 10m.
They can usually be found throughout the northern Atlantic and Pacific Oceans.
Their range extends from the ice edge in the Arctic during the summer to near the equator during winter.