LONDON (AFP) - Determined to show they would not be cowed by last week's deadly attack on the Boston Marathon, scores of Americans joined the hundreds of thousands of supporters cheering the London race on Sunday, where weary runners were urged to "finish for Boston".

The London Marathon prides itself on the carnival atmosphere along the 42.2km route, and this year's event, which kicked off after a solemn 30-second silence for the Boston victims, was no different.

Despite jitters following Monday's twin blasts in the US city, which killed three people and injured about 180, there were no barriers along the London course, so spectators were close enough to feel the breeze and smell the sweat from the passing athletes.

An estimated 700,000 people duly turned the streets of the British capital into a cheering corridor of support, with many determined to make a stand of solidarity with Boston.

Hanging from a pedestrian bridge over Upper Thames Street, about 5km from the finish line, a home-made banner told the tiring competitors passing underneath: "Run if you can, walk if you must but finish for Boston".

Mr Brian Kennedy, 40, a commercial lender from the Massachusetts city who has seen the Boston Marathon many times, noticed the sign and headed straight for it to lend his support. He was wearing an old Boston Fire Department t-shirt from the Boylston Street station - four blocks along from the marathon finish line.

"I'm from Boston, and I've been here for seven months on a work assignment," he told AFP.

"I saw this sign, and it put a big smile on my face," he said of the gesture. "It's special, it's really appreciated. It's unfortunate that it takes a situation like this to bring everybody together. I saw an article encouraging people to come out and cheer for the marathon, and I totally agree. I don't think anybody should be afraid. You're not going to get beaten by a few idiots."

Also on the bridge was Ms Katie Prahin, 35, a social worker who lives in London but hails from Virginia and went to college in Boston.

"It is good to be here today and not shying away. The silence at the start of the race was a nice way to show respect and support - it makes me happy that people have come out today," she said.

"You have to carry on with the way life is. You can't live in fear. After something a tragic as what happened, people have still come out. There's not tons of security but you know you're safe."

Down below, runners wore black ribbons to remember the Boston victims.

Amid the balloons and painted faces, spectators called out the fun-runners' names on their shirts to give them an extra push.

The more strained the athletes' faces, the louder the support.

Wheelchair athlete Justin Levene pulled up in the hope of getting a spare for his broken right wheel which was slowing him down from around 20 kmh to under four. But after a minute or so, and backed by a huge cheer, he said "Oh, sod it!" and resumed battle.

Spectator Nancy, from Bromley, in south-east London, said: "I've never been to see the marathon before, but I came because of Boston. It made me more determined to come, just to show support.

"Initially we all worried about coming out but once we thought about it, no, you can't let them beat you. And the atmosphere here has been fantastic."

Ms Anu Pohani, a New Yorker living in London, had four young children with her lining the route.

"I had a friend in Boston who got stopped from running because the explosions were taking place," she told AFP. "It's good that London got to go-ahead. They're not going to let anything happen here. Fear is not a factor."