LONDON (CNN) -- Despite the spring sunshine, the start of the London Marathon on Sunday was a somber occasion.
Thousands of runners joined in a 30-second silence on the start line to remember those killed and injured by the blasts near the finish of the Boston Marathon on Monday.
Before the tribute, signaled by a whistle blast, an announcer told the massed runners: "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 runners, many also wearing black ribbons as a Boston tribute, then poured over the start line to begin the 26.2-mile race.
Along the route, the mood became more festive, with loud cheers and applause for the passing runners from the spectators lining the barriers.
Prince Harry also made an appearance, visiting the finish line to show his support for the race stewards.
More than three-quarters of those taking part in Sunday's race are raising money for charity.
Race organizers are also donating 2 British pounds to the One Fund Boston -- set up to help those most affected by the Boston bombing -- for each runner to cross the finish line.
With about 35,000 runners, that's expected to come to about $100,000.
The first woman to cross the finish line was the winner of the Boston Marathon women's wheelchair race, U.S. athlete Tatyana McFadden, who repeated her feat at the London Marathon.
Ahead of the race, she told CNN she would be "racing for those in Boston and really carrying them in my hearts."
McFadden, who was born in St. Petersburg, Russia, spent her early years in a Russian orphanage -- walking on her hands because she had no wheelchair -- but embraced athletics after she was adopted and brought to the United States by her new family.
The winner of the men's wheelchair race was Australia's Kurt Fearnley, and the winning runner in the men's elite race was Tsegaye Kebede of Ethiopia.
The first-placed runner in the elite women's event was Priscah Jeptoo of Kenya. Fellow countrywoman Edna Kiplagat came in second.
The race is being staged amid heightened security, as police take extra precautions after the Boston Marathon bombing six days ago.
But race director Hugh Brasher urged participants in an e-mail Friday not to lose sight of what the London Marathon also stands for.
"One of the original aims of the London Marathon was 'to provide some fun and a sense of achievement in a troubled world,' " he wrote.
"Following the tragic events in Boston earlier this week, that goal will be even more meaningful when the running community comes together on the start line of this Sunday's race."
Mayor of London Boris Johnson tweeted to wish all those involved in the event good luck and "a great day out," adding, "@bostonmarathon our thoughts are with you today."
London's Metropolitan Police said there would be several hundred additional officers on the streets for the event, a move intended to "provide visible reassurance to the participants and spectators."
Police also appealed for anyone there to take extra care with their belongings, to avoid sparking security scares.
Hundreds of thousands of spectators usually turn out to support those taking part in the race.
Chief Supt. Julia Pendry, police commander for the event, said police worked with the organizers, partners and other emergency services to ensure the right plans were in place.
"Following the terrible events in Boston, we are providing additional visible reassurance to the public in what is naturally a worrying time," she said.
"I would stress there is no change to the threat level to London and nothing at this stage to link the Boston bombings to the London marathon."
Will Geddes, managing director of threat management company International Corporate Protection, told CNN this week that it is "very difficult" to secure a marathon because of the length of the route.
Any potential terrorist "will be looking for the largest number of casualties they can achieve, so the start point and the finish point will no doubt be two areas the Metropolitan Police will be focusing on and how they can secure those," he said. But, he added, "to a certain degree, there is only so much they can do."
The course, which starts in southeast London, wends through the Canary Wharf business district and passes by some of the capital's most famous landmarks, including Tower Bridge and Westminster, before finishing near Buckingham Palace.
Last summer, authorities implemented a huge security plan to keep the city safe during the London Olympics.
And they sought to reassure the public ahead of Sunday's race, with calls for supporters to come out in droves to show solidarity after the tragic events in Boston.
Sport Minister Hugh Robertson told the UK public broadcaster, the BBC, "The best way for us to react is to push ahead with the marathon, to get people on the streets and to celebrate it as we always do in London -- and to send a very clear message that we won't be cowed by this sort of behavior."
The London Marathon's chief executive, Nigel Bitel, also said, "It's a great occasion, the London Marathon, and I know that people will want to come out and send a message of support to runners on the day."