Security, black ribbons for Boston at London Marathon

Runners wearing black ribbons held a 30-second silence to honour the victims of the Boston bombings before setting off on the London Marathon on Sunday, under the watchful eyes of hundreds of extra police.

Around 36,000 runners were taking part in the London race, the first in the World Marathon Majors series since two explosions near the finish line of the Boston Marathon on Monday killed three people and wounded 176.

London's Metropolitan Police Service almost doubled the number of officers sent out to secure the event, saying this was to reassure the public and not a response to a specific threat.

The packed ranks of competitors bowed their heads and stood silently at the starting line, then clapped and cheered when a 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 including challenging two-person camel and horse costumes.

"Wishing all involved in London Marathon a great day out, good luck if you're raising money and Boston Marathon our thoughts are with you today," said Boris Johnson, the mayor of London, on Twitter.

After an unusually long and harsh winter, the weather came through for the marathon which began under bright sunshine and a cloudless sky, a bonus for the competitors and for the hundreds of thousands of spectators expected to cheer 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 enhanced security.

"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.

Before the race, competitors picked up their runner numbers and kit bags at the ExCel convention centre, where they were also provided with black ribbons.

At a message board inside the Excel, some runners had posted messages of solidarity such as "We'll be thinking of those in Boston" and "Praying for Boston".

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.