London Marathon runners have received a royal send-off as the Queen pushed the event's start button, sending thousands of competitors pounding through the capital's streets.
With clear blue skies and sticky highs of up to 23C (73.4F) expected, it is thought the 38th edition of the world-famous race could be the hottest on record.
The Queen started the event from Windsor Castle at 10am by pressing the traditional red button, seeing the elite men's race including four-time Olympic champion Mo Farah, sprint over line.
More than 40,000 marathon runners then proceeded to follow hot on their heels.
At 8.55am the men's and women's wheelchair racing began, with the World Para Athletics Marathon World Cup following at 9am and the women's elite racing kicking off at 9.15am.
In light of the warm weather, participants have been advised to drop their goal-times and organisers have added more ice, water and run-through shower stations along the 26.2-mile course.
Among this year's runners are firefighters who tackled the Grenfell Tower blaze, a police officer stabbed in the London Bridge terror attack and members of the Stephen Lawrence Charitable Trust.
Sunday marks 25 years since the murder of the teenager, who was a keen runner.
Meanwhile, almost 100 runners will attempt Guinness World Records - dressed variously in suits of armour, as mythical creatures and wearing stilts and ski boots.
Hundreds of police officers will be on duty to keep about 800,000 spectators and runners safe.
Competitors will start from Blackheath, south east London, running a snaking route along both sides of the Thames, finishing on The Mall near Buckingham Palace.
Last year, the event raised £61.5 million for charity, a world record for an annual one-day fundraising event, making the total raised since 1981 around £890 million, organisers said.
A record 386,050 people applied for this year's race - almost a third more than last year and the highest number for any marathon in the world.