Police had to be called to an outdoor swimming pool after hordes of overheating Londoners tried to force their way in.
Tempers flared around midday when waiting times topped three hours.
By around 1.30pm, a new record temperature of 36.9C had been set at Heathrow Airport.
The Metropolitan Police said: "Police were called to Brockwell Lido to reports of overcrowding.
"Officers attended. Security staff at the lido have closed the doors as a group of 500 people are trying to get in.
"The owners of the venue are advising people not to come as there is a three-hour waiting time."
Gauri Kangai posted a picture of the fracas on Twitter with the words "No City for Keeping Cool".
She added: "Raises (questions) about design in cities for extreme climate instances like today."
There were similar scenes elsewhere in the capital – with Parliament Hill Lido forced to refuse further entry just before noon due to overcrowding.
Police were repeatedly forced to attend the pool after fights broke out in the queue, and eventually the Met decided to leave an officer effectively on guard to prevent any more disruption.
The force said: "Officers were first called to the venue at 09.49hrs when a number of minor scuffles broke out as swimmers queued to enter the lido.
"Further incidents of disorder broke-out throughout the day."
A spokeswoman said there had been no arrests or any reported injuries, but added: "Police remain in attendance to prevent a breach of the peace."
In the South West, Portishead Lido in Bristol warned those hoping to cool off of very long queues.
Staff posted on Twitter: "Be prepared for a long wait, and there's no shade.
"Bring water, snacks, folding chairs, sunscreen, a hat and some patience & humour."
Lidos in Peterborough, London's Tooting Bec and at Hemsley in York were also forced to turn away disappointed swimmers after reaching capacity.
Forecasters predicted a high of 39C could be reached by the end of the day.
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