The capital’s streets were transformed into a sea of colour as a record 40,000 runners took part in this year’s London Marathon.
Prince Harry and the Duke and Duchess of Cambridge joined hands to press the traditional red button which started the elite men's and mass races on Sunday morning..
The royal trio, who champion the race’s official charity Heads Together, then dished out high-fives and took selfies with competitors about to pound the 26.2 mile course.
As runners began to pour over the finishing line in the afternoon sunshine, an act of comradeship between two competitors became one of the day's defining moments.
Swansea athlete Matthew Rees sacrificed his own race time to shoulder the weight of stricken David Wyeth, carrying his rival across the finishing line.
The 29-year-old said: "I saw him try to stand up again and his legs just went down again, and I thought 'this is more important, getting him across the line is more important than shaving a few seconds off my time'."
The images of the pair instantly drew comparisons to champion triathlete Alistair Brownlee who supported brother Jonny to the end of a race last year.
Among the famous faces who completed the arduous feat for good causes were radio personality Chris Evans and Olympic rowing gold medallists Heather Stanning and Helen Glover.
Former Top Gear host Mr Evans, who crossed the line for his third consecutive year to raise cash for Children in Need, said he had struggled to contend with the sunshine.
"It was definitely the toughest it had ever been," he said.
"I don't know why that was, I think it might be because it was hotter, maybe I didn't train as hard, but it was really much harder than it had ever been before.
"When it was cloudy over Greenwich Common it was freezing cold, then about half a mile in the sun came out and that was it, I had two tops and left one behind - it got hot really quickly."
On the prospect of a fourth marathon, he added: "If I'm alive, I'll be here."
For football presenter Mark Chapman, completing the London Marathon was just the start of his day.
He said: "I have to see first of all if I get back to Manchester and screw up Match of the Day 2 tonight, I tell you, I'm sitting down - I'm not walking to a touch screen or anything like that."
Earlier, 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.
Harry presented the elite runners with their trophies on the finishing line.
Hundreds of police were 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 watched over the crowds, while steel road barriers straddled the entrance to key roads along the route, including The Mall.
Organisers said a total of 40,382 people collected their race packs on Saturday, meaning the 37th event was set to outstrip last year's record of 39,140 competitors.
Among the runners battling the tarmac were a couple beginning an unconventional honeymoon, having tied the knot aboard the Cutty Sark on race day.
Jackie Scully, who has just completed treatment for breast cancer, wedded her partner Duncan Sloan on the historic ship just hours before they both crossed the start line.
World records were being attempted by about 50 runners, including the fastest marathon in a sleeping bag and fastest marathon in Wellington boots.
William, Kate and Harry said they hope it will be the year of the "mental health marathon" after their campaign, Heads Together, was selected as the London Marathon's official charity.
The races started at Blackheath and Greenwich Park from 8.55am and end in the shadow of Buckingham Palace.
Additional reporting by the Press Association.