Runners of all skill levels took to the streets around Belfast, with young and old out in force - many raising money for charity dressed in all manner of costumes.
One of those donning eye-catching garb was Gary Tweedie - who dressed up as Batman for the race.
Meanwhile in London Marathon runners were pounding the capital's streets today after earlier holding half a minute's silence in memory of those who died in the Boston bomb attacks.
Amid tightened security to reassure the racers and crowds, there was a 30-second silence just before the start of the men's elite race and mass start at 10am.
Among the many thousands watching the race was Prince Harry, who paid tribute to the "remarkable" way the people of Boston had dealt with the deadly attacks.
Harry said it was "never an option" for him to not attend the London event despite the two bombs which exploded near the finish line of the Boston Marathon last Monday.
Many of the 36,000 fun-runners, athletes and fundraisers wore black ribbons in a show of solidarity after three people were killed and 180 injured in the Massachusetts blasts.
Virgin London Marathon has pledged to donate £2 for every finisher in today's event to The One Fund Boston set up to raise money for victims of the explosions.
But organisers have stressed that as well as showing defiance and spirit in the showpiece event, the participants will have fun around the famous 26.2 mile course.
As usual the route was filled with runners in all kinds of fancy dress, including people dressed as a Roman soldier, Batman and Jack Sparrow.
Harry is making the presentations to the winners who include Ethiopia's Tsegaye Kebede who came first in the men's race and Kenya's Priscah Jeptoo who won the women's.
But Paralympic star David Weir said he was disappointed with his fifth place in the men's wheelchair race.
Speaking near the finish line, the six-time London Marathon champion said: "It was a tough race, but I knew it was going to be tough after four months out.
"I just had to do my best, and that's what I did today."
Britain's Shelly Woods also took fifth place in the women's event.
London 2012 double champion Mo Farah said he had to run to even get to the race after oversleeping this morning.
He only ran half the course as planned, as he said he wanted to concentrate on track running this year rather than long distance.
Earlier Geoff Wightman, the event commentator, announced over loudspeakers before the half-minute silence: "Marathon running is a global sport. It unites runners and supporters on every continent in pursuit of a common challenge and in the spirit of friendship and fellowship.
"This week the world marathon family was shocked and saddened by the events at the Boston Marathon.
"In a few moments a whistle will sound and we will join together in silence to remember our friends and colleagues for whom a day of joy turned into a day of sadness.
"Let us now show our respect and support for the victims of the tragedy in Boston."
The silence appeared to be perfectly observed.
The former director of the London Marathon David Bedford told the BBC: "We've had a fantastic response from our runners. There were many calls - please carry on, please run the race."