LONDON (AFP, REUTERS) - The London Marathon got under way on Sunday after marking a 30-second period of silence to remember the victims of the Boston Marathon attacks.

Elite and fun-runners alike, wearing black ribbons, stood in silence at the start line before setting off at 0900 GMT (5pm Singapore time). They halted their last-minute warm-up routines and stood still in the sunshine in Greenwich Park, many closing their eyes.

"We will join together in silence to remember our friends and colleagues for whom a day of joy turned into a day of sadness," the 35,000-odd runners were told over the public address system.

"Marathon running is a global sport. It unites runners and supporters on every continent, in pursuit of a common challenge and in a spirit of friendship and fellowship," the runners were told over the public address system.

"This week, the world marathon family was shocked and saddened by the events of the Boston Marathon.

"We will join together in silence to remember our friends and colleagues for whom a day of joy turned into a day of sadness.

"Let us now show our respect and support for the victims of the tragedy in Boston."

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 MR 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 42km 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 (S$3.80) 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).