About 37,000 runners showed up at the starting line in Greenwich for the race that snaked around the Thames River, ending at Buckingham Palace, Britain's The Sunday Telegraph reported.
Kelly Curran, a 23-year-old American from Boston now living and working in Buckingham, England, as a school lacrosse coach, ran the race wearing a T-shirt that said "I am Boston strong."
"I'm from Boston but I live over here and work over here. So I'm just showing my pride for the city. What happened Monday inspired me to bring my flag along with me," she said.
Curran said she was more worried about her legs lasting the race due to a knee injury than she was about her safety.
The event commentator, Geoff Wightman, called for a moment of silence in honor of the three dead and 180 injured in the Boston Marathon bombings before the start of the men's race, the newspaper said.
"Marathon running is a global sport. It unites runners and supporters on every continent in pursuit of a common challenge and in the spirit of friendship and fellowship," he said. "This week the world marathon family was shocked and saddened by the events at the Boston Marathon.
"In a few moments a whistle will sound and we will join together in silence to remember our friends and colleagues for whom a day of joy turned into a day of sadness."
Race organizer Virgin Group Ltd. promised to donate $3 for every runner who finished the race to a fund to support victims of the Boston blasts, the Telegraph said.