Some 36,000 runners were expected to take part in the London race, which also draws tens of thousands of spectators. Police said they planned to add 40 percent more officers and extra surveillance as a precautionary measure.
Most runners in London said they weren't worried by the Boston bombings, and the impressive turnout of enthusiastic fans lining the routes showed the same spirit.
Stuart Calderwood, an editor with a New York running magazine who has run in eight Boston Marathons, said the carnage there had made him and his friends more determined to run in London.
"We thought, 'What's going on with marathons? Are we vulnerable, in danger?'" said Calderwood, 55, after finishing the London course. "My group that came here, we just decided this is going to make us better. We're going to say marathons are the opposite of bombing and hostility and terror. People come from all over the world, work together to do something they couldn't do by themselves."
He said he put his hand on his heart as he crossed the finish line to honor the Boston victims: "I was thinking in the last mile about the kid that died, his name is Martin Richard and he used to run through every puddle he saw in the street. He loved to run. I ran that for him. ...This is for marathons and positive thinking."