The race began with a recorded countdown from British astronaut Tim Peake, who is currently on the International Space Station where he will run 26.2 miles (42.2 kilometers) on a treadmill.
The event is well-known for competitors donning costumes and this year’s race has seen a number of runners adopt a Shakespearean theme to mark the 400th anniversary of the bard’s death.
Runners also try to set world records during the event - 33 were set last year - and four members of the London Fire Brigade are attempting to establish the fastest time in a four-person costume when they set off dressed as a fire engine while another competitor will seek to set the record for running with a clothes dryer on his back.
Among the thousands of ordinary runners who run for charity - 54.1 million pounds ($77.9 milion) was raised last year - elite athletes from around the world were also competing.
Last year’s winners of the men’s and women’s race - Kenyan Eliud Kipchoge and Ethiopian Tigist Tufa - returned to defend their titles.
The race has been run every year since 1981 and 2016 will see the millionth runner cross the finish line.
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