London Marathon runners received a royal send-off as a record number began the gruelling 26.2-mile feat.
The Duke and Duchess of Cambridge and Prince Harry joined hands to press the traditional red button which started the elite men's and mass races.
The royal trio lent their support to the 40,000 competitors pounding the streets of the capital.
William, Kate and Harry champion the official charity of 2017's London Marathon - Heads Together - and earlier met racers competing for their campaign.
British wheelchair hero David Weir got the day off to a good start when he stormed to his seventh victory in the race as speculation rumbles over his retirement.
A world record was smashed by Kenyan Mary Keitany, who claimed her third London Marathon crown by beating Paula Radcliffe's women's-only world best.
Daniel Wanjiru then made it double glory for the African nation when the Kenyan athlete claimed first place in the men's elite race.
Hundreds of police are on duty to keep about 800,000 spectators and runners safe, barely a month after crowds were targeted in the Westminster terror attack.
Armed officers also watched over the crowds but the Metropolitan Police said: "There is no specific intelligence that the marathon will be targeted."
Steel road barriers straddled the entrance to key roads along the route, including the final stretch on The Mall, amid heightened concern of a vehicle-based assault.
Organisers said a total of 40,382 people collected their race packs on Saturday, meaning the 37th event is poised to outstrip last year's record of 39,140 competitors.
William, Kate and Harry said they hope it will be the year of the "mental health marathon", with blue headbands likely to be worn by many competitors this year in support of Heads Together.
The races started at Blackheath and Greenwich Park from 8.55am and end in the shadow of Buckingham Palace.