Big crowds lined the route of London's marathon on Sunday to cheer on about 36-thousand runners, many of whom wore black ribbons to honour the dead and wounded in the Boston bombings.
It was the first in the World Marathon Majors series since two explosions near the finish line of the Boston event last Monday killed three people and wounded 176.
Hundreds of extra police officers were deployed for the event, but the police said it was to reassure the public and not a response to a specific threat.
Reuters reports a 30-second silence was held in honour of the Boston victims just before the start of the race.
The packed ranks of competitors bowed their heads and stood silently at the starting line, then clapped and cheered when a whistle marked the end of the tribute.
Our correspondent in London says police were stopping people and searching backpacks, but the public were patient and cooperated fully with the heightened security.