Scotland Yard is not taking any chances, and has announced its plans for Sunday's marathon.
London's Metropolitan Police is putting 40% more officers along the route than it did last year, to reinforce security and in a bid to reassure the 36 000 runners and tens of thousands of spectators. Several hundred more officers, plus search dogs, will be deployed this year.
Runners have been encouraged to wear black ribbons and organisers have announced that £2 (about R28) for every finisher will be donated to a fund for victims of the Boston attacks. A 30-second silence will be held ahead of the elite men's race start.
London Marathon chief executive Nick Bitel said Saturday there would still be a "carnival atmosphere" around the course despite the heightened police presence. "After the terrible events in Boston we conducted a security review with the Metropolitan Police and all the other agencies," he told BBC radio. "We have increased substantially the number of police officers who are going to be out there, going to be visibly reassuring people and putting in more search regimes and a number of other measures to ensure that it is as safe and fun as it always is.
"I think people will come out and cheer and have a great day." The London course snakes alongside the River Thames and passes iconic landmarks including Tower Bridge, the Houses of Parliament and Buckingham Palace.
US police on Friday captured a young, ethnic Chechen man suspected of staging the Boston Marathon bombings, after a desperate manhunt that virtually paralysed the city and its suburbs. Responding to a tip from a local resident, police found Dzhokhar Tsarnaev, 19, hiding in a boat in a suburban backyard in Watertown, wounded and weary after a gun battle overnight in which his accomplice brother was killed. – AFP