The London Eye was closed on Wednesday and all but three Tube lines had speed restrictions imposed as temperatures in the city reached record heights.
People affected by the closure are entitled to a refund
Bosses at the attraction on London's south bank said they had shut the Millennium wheel after temperatures in the pods became too uncomfortable.
A passenger phone line said affected people were entitled to a refund or an alternative "flight".
Forecasters said temperatures in London reached 35C - making it the city's hottest day ever on record.
London transport bosses are closely monitoring temperatures on the Tube and have introduced speed restrictions on many lines.
Engineers are going out on the hour every hour to check the levels.
When the air temperature goes above 33C or the track temperature is 49C then a 30% speed restriction is imposed.
And if the air temperature reaches above 36C or the track temperature hits 53C then a 50% speed restriction is introduced.
On the Metropolitan Line a 25 mph speed restriction was introduced, the District Line was running at 20 mph and the Jubilee, Piccadilly, Central and Northern lines are among those affected.
A London Underground spokesperson said: "This may lead to a reduction in the number of trains running through evening peak, but we do not expect significant disruption to services.
Early morning swimmers enjoyed the warm weather by taking a dip in the Serpentine Lake at Hyde Park.
Tigers at London Zoo were fed lollies made of frozen blood
One told BBC London: "The water feels very refreshing, it's certainly quite warm at the moment and the hottest we've had this year."
At London Zoo the tigers were finding the heat too much and refused to feed apart from tasting some alternative lollies.
Zoo keeper Nicki McKay told BBC London: "They [tigers] do like the heat but obviously it's been very hot over the last few days.
"So we are giving them some ice lollies which are made from frozen blood. We use real blood."
London Eye customers can call 0800 0930123 for information