It is the first such sighting of the endangered species since records began nearly a century ago.
Onlookers crowded the Thames riverbanks as the mammal, about five metres long, swam upstream through the heart of the British capital past the Houses of Parliament and the London Eye Ferris wheel.
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 far upstream by Battersea.
Tony Woodley, of the British Divers Marine Life Rescue group, which is on standby to rescue the animal if it beaches, said several medics were at the site with rescue equipment.
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," he said.
"For an animal such as this to be where it is is very unusual and concerning," he said.