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."