Among the participants in London was Tomasz Hamerlak of Poland, who finished fourth in the men's wheelchair race and had competed in Boston last week. He said he was determined to race in London.
"It is terrible what happened in Boston, but we can't look back, we must look forward," an out-of-breath Hamerlak told The Associated Press moments after crossing the finish line. "The show must go on."
On Blackheath, the spacious green common area where the race begins, runners massaged one another's legs as loud pop music boomed on a sound system. A half-dozen police officers in reflective vests strolled around and chatted with the runners.
Moments before the majority of runners set off on the grinding course, announcer Geoff Wightman used the loudspeakers to ask for silence. He described marathon running as a global sport that unites runners and supporters in every continent in a spirit of friendship.
"This week the world marathon family was shocked and saddened by the events at the Boston Marathon," he said as he asked the people gathered to "remember our friends and colleagues for whom a day of joy turned into a day of sadness."
As those gathered responded to his call, the only noise that could be heard was the buzz of helicopters and the beeping of a truck.
Security was plentiful but not intrusive near the finish line at the Mall in front of Buckingham Palace. Marathon staff, officials and media had their bags thoroughly checked, a process not deemed necessary at the event last year. Officials said this was in response to the Boston attack.
Shirley Gillard, a 63-year-old retiree sitting on a bench at the edge of the starting area, seemed pleased with her decision to come out and watch the race. She described herself as the type of person who was always worried when spotting an unattended bag on public transport, but said people shouldn't change their habits because of what happened in Boston.