Undaunted by the Boston Marathon bombings, big crowds lined the route of London's mass road race on Sunday to cheer on around 36,000 runners, many of whom wore black ribbons to remember the dead and wounded.

Hundreds of extra police were deployed to secure the first race in the World Marathon Majors since two explosions near the finish line of the Boston Marathon on Monday killed three people and wounded 176.

"It is a bit different this year because everybody's aware of what happened in Boston... They're not going to stop us running," said Steve Williamson, a three-time competitor in the London Marathon now taking part as a marshall.

"It was incredible, the amount of support, people coming out from everywhere, just cheering the whole way. Unbelievable," said a breathless Mo Farah, Britain's 5,000 and 10,000-metre Olympic champion, after running the first half of the course.

Farah ran half the route to prepare for competing next year.

London's Metropolitan Police Service said it increased the number of officers on the streets to reassure the public and not in response to any specific threat.

The enhanced security did not dampen the party atmosphere, with marathon regulars saying the turnout was bigger than in previous years and a brass band near the starting line jokingly complaining the cheering crowds were drowning out their music.

Before the start of the men's elite and mass races, official commentator Geoff Wightman led the crowd in a tribute to Boston.

"This week the world marathon family was shocked and saddened by the events at the Boston Marathon," he said over loudspeakers.

"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."

VISIBLE REASSURANCE

The packed ranks of competitors bowed their heads and stood silently for 30 seconds, then clapped and cheered when a second whistle marked the end of the tribute.

Seconds later, the world's elite runners led off the race. Behind them came thousands of competitors chasing personal goals or raising money for charity, many running in fancy dress.

Unusual outfits included a two-person camel costume, a giant beer bottle, two male brides and Admiral Lord Nelson.

After an unusually long and harsh winter, the weather came through for the London Marathon which unfolded under bright sunshine and a cloudless sky, a bonus for the competitors and for the hundreds of thousands of spectators cheering them on.

The 26-mile course starts in leafy Greenwich, crosses Tower Bridge, snakes through the Canary Wharf business district before going through the heart of London, past Big Ben to Buckingham Palace.

Police with sniffer dogs were out in force and bins had been removed from the length of the course as part of a security boost.

"The enhancement to policing, which will see several hundred additional officers on the streets, is intended to provide visible reassurance to the participants and spectators alike," the Metropolitan Police said on its website.

There was a 40 per cent increase in officers on the street compared with what was planned before the Boston bombings.

The organisers will donate 2 pounds per finisher to The One Fund Boston, set up to raise money for the victims. They estimate around 35,500 people will cross the line, meaning they are likely to raise at least 70,000 pounds ($107,000).

In the German city of Hamburg, which was staging its own marathon on Sunday, runners also wore ribbons and held a minute of silence for victims of the Boston bombings. – Reuters