More than 37,800 people are running and walking the 26.2 mile course of the London Marathon from Blackheath to the Mall: the biggest in the event's history.
Paula Radcliffe, who set the current world of 2 hours 15 minutes 25 seconds in April 2003, is running her final race.
Speaking to BBC Sport, Radcliffe said: "I want to do myself justice, so obviously I won't be close to my fast time.
"It will be a totally different experience for me."
Many competitors are raising money for charity and aiming to suppass 2014's record amount of £53.2m.
The event kicked off with a mini marathon for young people aged between 11-17 years, before the wheelchair race, where home favourite David Weir was hoping to land a record seventh crown.
However, he finished second, beaten by American Joshua George. Fellow American Tatyana McFadden has won the women's wheelchair race.
In the elite men's race Wilson Kipsang is hoping to become the fourth man to win three titles.
Many celebrities are running including Formula 1 star Jenson Button and Radio 1 DJ Greg James.
Athlete David Hemery, who won gold in the 1968 Olympic Games in the 400m hurdles, is running his first marathon for 39 years at the age of 70.
The mother of the cancer fundraiser Stephen Sutton is also running her first London Marathon to keep a promise she made to her son before he died.
Jane Sutton is hoping to raise £10,000 for the Teenage Cancer Trust.
She said: "I am fulfilling my wish to Stephen, he has been my driving force and when I have been going out running he has been the person I am thinking about and hopefully I've got the same determination as him so I will cross that finish line."
The oldest entrant is Paul Freedman, 90, from Hornchurch, Essex, while the youngest will be Jonny Innes from Falmouth, Cornwall, who is celebrating his 18th birthday.