London (CNN) -- Runners and spectators will see heightened security at the London Marathon on Sunday, as police take extra precautions to allay fears in the wake of the Boston Marathon bombing six days ago.
The start of the race will be a more somber occasion than usual.
A 30-second silence is being held to remember those killed and injured by the blasts near the finish line of the Boston race on Monday. The 35,000-plus runners in London also are being encouraged to wear black ribbons as a mark of respect.
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."
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.
Chief Supt. Julia Pendry, police commander for the event, said the police had worked with the organizers, partners and other emergency service to ensure the right plans are 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 earlier 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 26.2 mile 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, St Paul's Cathedral and Westminster, before finishing near Buckingham Palace.
More than three-quarters of those taking part will raise money for charity.
And the organizers of the London Marathon have now pledged to donate £2 ($3) to The One Fund Boston for every runner who crosses the finish line.
Last summer, authorities successfully implemented a huge security plan to keep the city safe during the London Olympics.
And they've sought to reassure the public ahead of Sunday's race, with calls for supporters to come out in droves to show solidarity following 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."
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."
Elite wheelchair racer Tatyana McFadden, who won the women's wheelchair division in her first Boston Marathon on Monday, told CNN she was "really looking forward" to racing in London -- despite the chaos and fear she witnessed less than a week ago.
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 her life improved dramatically after she was adopted and brought to the United States by her new family.
"On Sunday I'll be racing for those in Boston and really carrying them in my hearts," she said.