Police divers are searching for three people missing in different stretches of the River Thames.
They are looking for a 22-year-old man who failed to resurface after going swimming with friends in Shadwell Basin, Wapping just after 18:00 BST.
Officers are also searching a stretch of the river at Waterloo bridge after reports of a person in difficulty in the water.
A third person was reported to be in the water at Kingston upon Thames.
As temperatures in London reached 32C, thousands of people headed for the river.
In east London, divers searched for a man who disappeared under the water while with fellow swimmers.
A Metropolitan Police spokesman said: “Police were called… at 18:06 to Garnet Street (Shadwell Basin) to a report of a man seen to enter the water.
“The man, believed to be aged 22 years, was swimming with friends and has not resurfaced.”
Several hours after the search began the man was still missing.
Pictures posted on Twitter showed large numbers of people relaxing in the sun and on the water, in canoes in soaring temperatures.
At 20:30 police were called to central London, where a person was reported missing in the river.
Shortly after, at 20:35, more officers attended at Kingston, after reports another man was seen in the river near High Street.
The Met said the Marine Policing Unit and RNLI led the search.