Downward dogs and yoga mats have replaced cars and buses on London’s Tower Bridge as part of Car Free Day.
The mass yoga session was one of a number of activities taking place in the capital as more than 16 miles (27 km) of streets were shut.
Bank junction has been turned into a festival space while children will race go-karts in the Square Mile.
The closures will be in place until 19:00 BST with roads elsewhere expected to be busy as a result.
Tower and London Bridge were shut at 07:00 along with streets in parts of the City, Southwark and Tower Hamlets.
Among the other activities taking place are a hedge maze in Cheapside and classic cycle rides on Tower Bridge.
Organisers hope more than 150,000 people will join the event which has been named Reimagine.
Away from the centre, 15 boroughs will be running their own Car Free Day celebrations and more than 340 “play streets” – safe spaces for local people to socialise and play – have been approved some 24 boroughs.
London’s Mayor Sadiq Khan said the day was about “demonstrating our commitment to cleaning up our toxic air and experiencing a greener way of living”.
Transport for London has warned that those who do take to the roads should expect “significant delays”.
The UK has had its hottest day of the year, as temperatures soared across southern England.
The Met Office said Heathrow and Northolt in west London had reached 34C (93.2F) making it one of the warmest June days for about 40 years.
Friday was previously the warmest day of 2019, with temperatures reaching 30C (86F) at Achnagart in the Highlands.
A heatwave across Europe saw France record its all-time highest temperature of 45.9C (114.6F) on Friday.
People flocking to the seaside were forced to take detours after the M5 was closed in Somerset in both directions because of problems with overhead power cables.
At Glastonbury Festival in Somerset, the temperature was expected to peak at 28C (82.4F), with organisers giving away free sun cream and water to help combat the heat.
Festival-goers reported long queues, with freelance journalist Sara Spary saying it took almost an hour to refill her water bottle.
Organisers said there was “no water shortage” and the supply was “running as normal”, although it has put restrictions on showers as it usually does in hot weather. It said its 850 taps all have a ready supply of water.
The Met Office has issued yellow warnings of thunderstorms and lightning for parts of northern England and south-east Scotland for Saturday evening.
Temperatures are expected to drop overnight across the UK as cold air moves in from the Atlantic, sweeping away the humidity, said BBC forecaster Matt Taylor.
He said: “There won’t be the same humidity on Sunday but there will be sunshine at times and it will feel quite pleasant.”
He added that while central and eastern parts of the UK saw the hottest and most humid conditions on Saturday, the sunshine gave way to some storms in Northern Ireland.
“These are now pushing their way eastward to cross Scotland and the far north of England.”
Earlier, England’s most senior nurse called on people to “check in on neighbours and loved ones who can suffer the most from heat and pollen”.
Ruth May, chief nursing officer for England, also said it was important to drink plenty of water, use a high-factor sunscreen and take allergy medication if you need it.
NHS England and emergency services have also warned the public to take extra care.
It follows the death of 12-year-old Shukri Yahya Abdi, who drowned in the River Irwell in Greater Manchester on Thursday.
And animal charity the RSPCA issued advice for pet owners to help them keep their animals cool.
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