One suspect in the Boston bombing was killed in a city suburb on Friday and a manhunt was underway for a second suspect.
Two explosions near the finish line of the Boston race killed three people and injured 176, triggering security scares across the United States and prompting British authorities to review security arrangements ahead of the London event.
The head of the Metropolitan Police's London marathon operation, Julia Pendry, said on Friday there would be several hundred more officers on the streets than had originally been planned for Sunday's race.
"We've got more search dogs out, we've got more officers on high-visibility patrols," she told reporters gathered outside the Metropolitan Police headquarters at New Scotland Yard, less than two kms from the marathon's finish line.
The increased policing was intended to reassure the 36,000 runners and the hundreds of thousands of supporters who were expected to line the course as the race wound round Tower Bridge and the Canary Wharf financial district before ending in front of Buckingham Palace, she said.
"There is no change to the threat level at this time to London," she said. "There is no link at this time between the Boston marathon and the London marathon, so come out and enjoy the day as normal."
Pendry, who has worked on the police operation for the London marathon for five years, said that officers were well briefed and many contingency plans were in place to protect the 42-km route.
She said the Metropolitan Police had been in touch with security authorities in Boston regarding the event.
Westminster Council said in an emailed statement that in its borough, the site of the race's finish, all rubbish bins near the course were being removed as a security measure.
- Athletics, Track & Field