Police on motorbikes ride past advertising balloons at the start line of the London Marathon in London on April 21. Runners at the London Marathon on Sunday wore black ribbons and observed a 30-second silence to honour the victims of the Boston Marathon bombings, under the watchful eyes of increased numbers of police deployed to reassure the public. Some 36,000 runners are expected to take part in the London race, the first in the World Marathon Majors series since two explosions near the finish line of the Boston race on Monday killed three people and injured 176. Reuters/Toby Melville