Thousands of runners at the London Marathon, including Olympic champion Mo Farah, observed 30 seconds of silence at the start of the race, which took place less than a week after the Boston attacks.
The poignant tribute was followed by applause as the runners gathered at the start line for the elite men's and mass race with large crowds lining the streets of the UK capital under heavy security.
Many of the 36,000 fun runners, athletes and fundraisers wore a black ribbon as a mark of respect to those affected by the attack.
Some had the name of the US city emblazoned on their vests, while some spectators held up banners saying "For Boston".
Tsegaye Kebede of Ethiopia won the men's race in , while Priscah Jeptoo of Kenya won the women's event.
Huge crowds turned out to watch the races on a glorious spring day in London despite concerns raised by the bomb attacks on the Boston Marathon.
"It was incredible, the amount of support, people coming out from everywhere, just cheering the whole way. Unbelievable," said a breathless Farah.
Prime Minister David Cameron wrote on Twitter: "Congratulations to the inspiring runners in the #LondonMarathon - a fantastic showcase for the city."
Prince Harry, who waited at the finish line on the Mall in front of Buckingham Palace to hand out medals to the winners, said the large turnout was "fantastic" and "typically British".
He also paid tribute to Boston, saying: "The way that Boston has dealt with it has been absolutely remarkable.
"It's never going to get anyone down here, the great thing about the marathon is no matter what colour you are, or religion, no matter what nationality you are, everyone comes together to run a certain distance to raise money for amazing causes.
"I think that you can never that take away from people," he told the BBC.
Virgin London Marathon has pledged to donate £2 for every finisher to The One Fund Boston , which was set up to raise money for the victims of the explosions.
Three people were killed in Boston, including an eight-year-old boy, and over 170 injured, some seriously.
Following the attacks, hundreds of extra police were deployed in London - a 40% increase in the usual number of officers.
Authorities stressed the increased security was aimed at reassuring the public and was not a response to a specific threat.
Despite the sombre tributes and the increased police presence, the mood was a festive one, with spectators cheering the runners on and some competitors showing up in fancy dress.
"It means that runners are stronger than bombers," said Valerie Bloomfield, a 40-year-old participant from France.
Among the runners was Stuart Singer, who ran the Boston Marathon on Monday but crossed the finish line ahead of the blasts. He said there was never any doubt he would run.
Mr Singer, whose wife had been standing on the spot the second bomb was detonated 15 minutes before the explosion, said: "I just didn't feel there was any risk on the streets at all."
Barbara Stephenson, charge d'affaires at the US Embassy in London, told Sky News the show of respect by London Marathon runners for the Boston victims underlined the "special relationship" between the two countries.
"We've had responses from Her Majesty the Queen, all through Twitter from the British people, and now we have got tens of thousands of London Marathon runners wearing a black ribbon in solidarity with the people of Boston."
She went on to say that "it's moments like this when you know what the special relationship's really all about".
The winding route from Blackheath to Buckingham Palace took the runners past some of the capital's most famous landmarks.
Farah, the Olympic 5,000m and 10,000m champion, ran about half the race, stepping off the course at the 13.1 mile mark near Tower Bridge.
He revealed before the event that he had overslept. "I'm late! I woke up late! I'm going to miss the bus!" he joked.
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."
Celebrities running this year included the drummer from the pop group McFly, Harry Judd, the shadow chancellor, Ed Balls, and the singer Katherine Jenkins. Speaking on the finish line, Mr Balls said that the Chancellor, George Osborne, had sponsored him.