The London Marathon runners fell silent on the start line on Sunday 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.
Many of the 36,000 fun-runners, athletes and fundraisers wore black ribbons in a show of solidarity to those affected by the terror strikes at the finish of the Boston Marathon last Monday.
The men's race was won by Ethiopian Tsegaye Kebede in a time of 2:06:04 whilst Kenya's Priscah Jeptoo won the women's race in 2:20:15.
Britain's David Weir finished fifth in the wheelchair race failing to clinch a seventh win.
Three people were killed and 180 injured in the Massachusetts blasts.
The senior US diplomat in the UK said the show of respect by London Marathon runners for the victims of the Boston terror attack underlined the "special relationship" between the two countries.
Charge d'Affaires Barbara Stephenson thanked Britons for their show of support in the wake of the attack at the finish of the Boston Marathon last Monday.
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.
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.
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.
Earlier 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."
Shadow chancellor Ed Balls said it was important to show that Britain would "carry on regardless" despite the Boston bombing.
He told LBC 97.3 that his wife, shadow home secretary Yvette Cooper, thinks he is "completely mad" for running a second time, despite insisting "never again" after last year's race.
Balls, who is raising money for the charities Action for Stammering Children and Whizz-Kidz, said: "In politics sometimes doing a U-turn is the wise thing to do."
Katherine Jenkins, the Welsh singer, said she was running in memory of her father and to show solidarity to people affected by the bombings in the US.
"Like everyone else, it was just so devastating to see that on the news," she said.
"But I think we all feel more than ever that we want to do this and show our support for Boston."
The 32-year-old said she has raised over £22,000 for a cancer care charity.
She said: "I wanted to run it in support of Macmillan. My father passed away from cancer when I was 15 and the Macmillan nurses came and they were wonderful."
She added: "I really don't think of myself as an athletic person at all.
"I never in a million years thought I would do this. I'm glad that I am, but I am nervous."
Extra security has been in put in place around the bags of the runners, the race's baggage manager said this morning.
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Phil Keith said that 34 articulated lorries would take the rucksacks of the thousands of runners from the start in Blackheath in south east London to the finish on the Mall near Buckingham Palace in the centre of the capital.
The bombs which went off in Boston are believed to have been placed in rucksacks placed on the route.
"It (security) is a problem but we have taken a lot of advice from the Metropolitan Police," he told the BBC.
"We have increased the security around the baggage.
"It is all very safe. Everyone will be incredibly safe at the finish and at the start."
Prince Harry, who is making the presentations for the event this year, walked along The Mall to meet the staff and volunteers.
People waved and cheered, and Harry stopped to pose for pictures.
Cate Scot, 56, who is volunteering at the baggage reclaim trucks, met the 28-year-old prince.
"It was a great honour for me to meet him, and I appreciate the support he gives to the volunteers," she said.
"Last year he passed by me, and I really wanted to thank him for giving us this boost.
"So this year I managed to say thank you so much for coming."
Scot, an athletics coach with Barnet and District Running Club, who has volunteered at the London Marathon for the last nine years after competing in 2003, said: "It's vital, the support the volunteers give.
"For me, the volunteers are as important as the competitors."