"The best way for us to react is to push ahead with the marathon, to get people on the streets and to celebrate it as we always do in London -- and to send a very clear message that we won't be cowed by this sort of behavior," Sport Minister Hugh Robertson had earlier told the BBC.
There was also a strong police presence with 40% more officers on duty in the wake of Monday's bombings. As an extra security measure, litter bins were also removed from the 42-kilometer route.
This is the city which nine months earlier hosted the Olympic Games without incident, and politicians say it provided the ideal training ground for hosting international sporting events.
At the heart of the competition were ordinary people slogging it out in one of the most grueling races of its kind in the world -- and all for charity.
As British journalist Marina Hyde wrote this week: "The Boston bombers served only to remind the world that marathons are something in which we all have a stake."
The message, on this sundrenched Sunday, was loud and clear: "We are in it together."
The first woman to cross the finish line was the winner of the Boston Marathon women's wheelchair race, U.S. athlete Tatyana McFadden, who repeated her feat at the London Marathon.
The winner of the men's wheelchair race was Australia's Kurt Fearnley, and the winning runner in the men's elite race was Tsegaye Kebede of Ethiopia.
The first-placed runner in the elite women's event was Priscah Jeptoo of Kenya. Fellow countrywoman Edna Kiplagat came in second.