The whale was spotted at around 2 p.m. ET (7 p.m. local time) Sunday stuck near Richmond Lock on the River Thames, a port spokesperson told CNN.
The whale is believed to be a small minke whale approximately 3 meters long, he said.
Marine mammal experts from the British Divers Marine Life Rescue have arrived on the scene and are working to free the whale.
The experts have been joined by authorities from Port of London Authority, the Royal National Lifeboat Institution and the London Fire Brigade, the spokesperson said.
Spectator Daniel Magee, who recorded several videos of the whale, said he assumed it was a returning seal.
“The location is Richmond Lock and and we saw a group of spectators, we saw a seal there earlier this year, which is unusual, so we thought it was him again,” Magee told CNN.
“As we got closer I saw a fin and realized it was a whale. Some lock keepers hosing him down and it looked like the tide was going up so he could turn around. I realized that the whale might be injured as it started rolling on its side and thrashing about.”
Another spectator, David Korsaks, told CNN he was surprised to see anything other than birds in the area.
“It was almost disbelief and shock to see a whale where you would normally only see ducks and swans,” Korsaks said. “My next thoughts were I hope it’s ok and manages to swim free.”
Sophie Milner told CNN she took video of the whale after seeing people gathered at the scene.
“We just saw a crowd of people looking at the whale. It was being looked after by some specialists by the time we got there,” she said.
CNN’s Susannah Cullinane contributed to this report.