Thousands of runners are set to take on this year's London Marathon, including one competitor who will tackle the course from space.
As participants prepare to pound the capital's streets, British astronaut Tim Peake will aim to complete the route while strapped to a treadmill at the International Space Station.
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.
The 44-year-old has recorded a good-luck message, which will be shown to his fellow runners on a big screen, and ends with a 10-second countdown.
In another milestone for the race, which is in its 36th year and for which more than 39,000 people have been allocated places, the one millionth competitor will cross the finish line on Sunday.
A host of celebrities are signed up to run for various good causes, including Game of Thrones star Natalie Dormer, who is running for 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.
Double Olympic champion Dame Kelly Holmes will make 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 at this year's Virgin Money London Marathon.
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 start and spectators are being urged to wrap up warm with a high of just 12C (54F) forecast for the day.
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.
Scores of entrants will attempt 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 will 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 will all lace up their trainers again for the big day, while Rochdale MP Simon Danczuk and Tory Amanda Solloway will make their debuts.
The races start at Blackheath and Greenwich Park from 8.55am and end on The Mall.
Road closures will be in force and there will be delays to some public transport.