Thousands of people are taking part in the London marathon, after a countdown from space by astronaut and competitor Tim Peake.
The British spaceman, who is running while strapped to a treadmill at the International Space Station, sent his fellow runners a good luck video message before a 10-second countdown to start the 36th race.
Major Peake, who ran the 26.2-mile course in three hours, 18 minutes and 50 seconds on earth back in 1999, will see the roads pass under his feet in real time on an iPad.
In another milestone for the Virgin Money London Marathon, which more than 39,000 people are thought to have started, the one millionth competitor will cross the finish line on Sunday.
A host of celebrities are running for various good causes, including Game of Thrones star Natalie Dormer, whose chosen charity is the NSPCC, and Top Gear host Chris Evans, who is taking on the challenge for the second year in a row, raising money for Children in Need.
Evans, who managed to complete the race in four hours and 53 minutes in 2015, said he is looking forward to doing it all over again.
Describing himself as "pretty chilled" after having had a good night's sleep, he told BBC Breakfast: "I'm more excited than last year, which I didn't think I could possibly be."
Comparing his day job driving fast cars for Top Gear with running the marathon, he said: "That's far easier than this, but not as fulfilling to be honest."
Double Olympic champion Dame Kelly Holmes is making her London Marathon debut as she aims to raise a quarter of a million pounds for a number of charities including one she founded to help disadvantaged young people.
There are 70 years between the oldest and youngest runners in this year's race.
Race veteran Iva Barr, 88, from Bedford, said the day feels "like being at the centre of a big street party". She will run her 20th London Marathon, raising money for disabled young people's charity Whizz-Kidz.
Rebecca Manners from Manningtree in Essex will celebrate her 18th birthday by running the route alongside her parents, in aid of VICTA, which supports blind and visually impaired children.
This year's runners face a chilly day and spectators are being urged to wrap up warm with a high of just 12C (54F) forecast.
Snow had been predicted for the first time in the event's history but that is unlikely to materialise, the Met Office said.
In the elite men's race, Kenyan Eliud Kipchoge will aim to defend his 2015 title after he pipped former champ Wilson Kipsang to the finish.
Meanwhile, in the elite women's race, Tigist Tufa will be hoping for a repeat performance of her thrilling finish last year. The Ethiopian outran the favourites to take the 2015 title.
Prince Harry, patron of the London Marathon Charitable Trust, will present the winners' prizes.
The marathon's chosen charity for next year was announced on Sunday as competitors set off.
Heads Together, which will take over from the NSPCC, is made up of a team of mental health charities which have been brought together by The Royal Foundation of the Duke and Duchess of Cambridge and Prince Harry.
Scores of entrants are attempting to break 60 Guinness World Records this year, including the fastest marathon by two runners handcuffed together, and another for a man running in riot gear.
Eight MPs signed up to compete in the race, which attracts elite athletes, amateur runners and fundraisers each year.
Conservatives Alistair Burt, Alun Cairns, Graham Evans, and Edward Timpson as well as Labour's Dan Jarvis and Jamie Reed are all running again while Rochdale MP Simon Danczuk and Tory Amanda Solloway will make their debuts.
Runners set off from Blackheath and Greenwich Park, running a route lined with supporters, and will finish on The Mall.
Road closures will be in force and there will be delays to some public transport.