With experts indicating there could be some heavy showers through the morning, rainstorms are keeping racers cool.
Today is the London Marathon, which is expected to exceed all previous records for the largest marathon ever with more than 50,000 participants.
Elite runners are at the front of the pack, and thousands are running the gruelling 26.2-mile route through the capital for charity.
Just some of the famous faces on the starting line include the actor Stephen Mangan and the TV presenter Mark Wright, who was forced to drop out last year due to injury.
Outbreaks of rain are set to keep competitors cool, with forecasters warning there could be some heavy showers through the morning.
Temperatures of about 13C (55F) are expected at the start of the race – rising to 17C (62F) by the middle of the afternoon.
Met Office meteorologist Steven Keates encouraged spectators to bring an umbrella “as the weather may not be the best when standing around”.
Competitors and their families had been warned of disruption this morning because some trains were starting later than normal owing to yesterday’s rail strikes.
But services to Blackheath – the station closest to the start line – were expected to run as normal.
Among those taking part will be Rob Duncombe, chief pharmacist at The Royal Marsden NHS Foundation Trust.
He is hoping to raise thousands of pounds for The Royal Marsden Cancer Charity while setting a new world record for the fastest marathon dressed as a tree.
Kitted out in a 2.4m (8ft) tall oak tree costume, Mr Duncombe is supporting the charity’s Oak Cancer Centre appeal, which aims to develop a new treatment and research facility in Sutton.
He told Sky News: “When the opportunity came to run the marathon for the Royal Marsden, just to add a little bit of extra challenge, I said I’d run it as a tree for us.”
The current men’s world record for the fastest marathon dressed as a tree stands at four hours, five minutes, and six seconds.
Mr Duncombe is aiming to run the marathon in a time of four hours, despite his costume being “seriously uncomfortable”.
Hugh Brasher, the London Marathon’s race director, told Sky News that the virtual marathon introduced in 2020 will run again this year – allowing thousands around the globe to participate. There’s going to be a mini-marathon for children, too.
This year’s course also features a 250m stretch called Rainbow Row, celebrating the LGBTQ+ community at the 21st mile.
Mr Brasher said it will create a carnival atmosphere, to give runners “that extra boost” as they approach the last five miles.
He added: “The marathon might be painful at times… it probably will be.
“But that spirit, the crowd, you’re going through this amazing capital city and you’ve got maybe three quarters of a million people shouting and cheering you.
“It is an incredible, emotional, heartfelt experience, lap it up.
“Enjoy it.”
This will be the third and last time the event is held in October.
The London Marathon will return to its traditional April slot in 2023 – and ballots for that race opened yesterday.