Kenya's Priscah Jeptoo took advantage of a dramatic fall by Olympic champion Tiki Gelana to win the London Marathon.
After Gelana was involved in a collision with Canadian wheelchair racer Josh Cassidy, Jeptoo won in two hours, 20 minutes and 15 seconds.
In the men's race, Ethiopian Tsegaye Kebede overhauled 2011 champion Emmanuel Mutai in the closing stages for his second win in 2:06:04.
A 30-second silence was held in tribute to victims of the Boston bombings.
In addition, competitors were encouraged to wear black ribbons.
Those terrorist attacks at the end of the world's oldest marathon had taken place only six days before the 33rd staging of the London race.
As a result, extra police officers - 40% more than last year - were deployed to ensure safety.
Yet, any question that events in Boston might drive fans away was dismissed as supporters lined the streets of the capital on a sunny Sunday morning.
Indeed, conditions were so ideal for running that it was suggested that a world record may have been broken in one of the elite races.
And record pace was being kept in the men's race up to about the halfway mark, where Great Britain's double Olympic champion Mo Farah
From there, and with the record being a more unlikely prospect, Mutai built what looked to be a winning lead.
But, with around 1km remaining, Kebede, who was controversially omitted from the Ethiopia team for the 2012 Olympics, breezed past unchallenged to repeat his London success of 2010.
Ethiopian Ayele Abshero finished third, with Olympic champion Stephen Kiprotich of Uganda sixth.
Earlier, the women's elite and men's wheelchair races converged at a water station, ending the challenges of both Gelana and Cassidy.
Gelana, part of the leading pack at the 15km mark, cut across the path of Cassidy as she attempted to collect a drink.
The Ethiopian fell and her leg appeared to go under Cassidy's chair. She rejoined the race, but faded in clear discomfort.
With Gelana out of contention, Jeptoo, the 2012 Olympic silver medallist, pulled clear to take victory ahead of compatriot Edna Kiplagat and Japan's Yoko Shibui.
"Today I'm very, very happy, I couldn't believe I could be the winner," Jeptoo told BBC Sport. "It is a very tough race because everybody who comes here is really prepared."
Great Britain's Susan Partridge came ninth in a time of 2:30:46, enough for a place at the World Championships in August.
In the wheelchair event, the crash ended Cassidy's participation in a race that was won by Australia's Kurt Fearnley. He emerged from the pack in a sprint finish that left the tiring British six-time winner David Weir trailing in fifth.
"It's something I have mentioned before," said Cassidy. "I don't know who's responsible, but every year we come to overtake the women, there's 10 chairs going at 20mph and the poor women are scrambling to find their feet.
"I have a brand new $2,000 pair of wheels that are damaged, who's going to pay for them? Things have to change.
"The safest thing would be to have the chairs start first because one of these years a woman is going to have a leg broken, a career ruined. It's just not worth having this programme if the races are going to suffer."
American Tatyana McFadden emerged victorious in the women's wheelchair race, with Britain's Shelly Woods fifth.