Media playback is unsupported on your device Media caption Runners and spectators observe a 30-second silence

Tens of thousands of runners have taken part in the London Marathon, many with black ribbons on their vests in honour of the victims of the Boston bombings.

A record 700,000 spectators lined the streets as the 36,000 runners set off from Blackheath, where a 30-second silence was held.

Three people died and more than 170 were injured by Monday's blasts, near the finish line of the Boston Marathon.

Hundreds of extra police officers were drafted in as reassurance in London.

About half a million people were expected to watch the race, although official estimates have yet to be made.

Just before the start of the main race, event commentator Geoff Wightman introduced the half-minute silence.

At the scene The determination of those struggling to keep their feet moving as the finish line in The Mall approaches is touching to witness. The crowds' cheers spur them on. There are many guises to the post-marathon posture. There are those who bend over, hands firmly on knees as they try to control their breathing, some do cooling down stretches, some whoop with a sense of achievement. Others look at their watches time and time again to check their finishing time. Some runners let themselves be gently led to help by the medical team. The less fortunate are taken away by stretcher without anyone so much as batting an eyelid in a way that could only happen in a big marathon event like this.

"Marathon running is a global sport," he said.

"It unites runners and supporters on every continent in pursuit of a common challenge and in the spirit of friendship and fellowship."

He said the marathon family had been "shocked and saddened" by the events in Boston and the silence was to show "our respect and support for the victims".

A senior US diplomat based in the UK said the commemoration underlined the "special relationship" between the two countries.

Barbara Stephenson, Charge d'Affaires at the US Embassy, told Sky News: "We've had responses from Her Majesty the Queen, all through Twitter from the British people, and now we have got tens of thousands of London Marathon runners wearing a black ribbon in solidarity with the people of Boston.

"As my senior law enforcement person said on Friday afternoon, it's moments like this when you know what the special relationship's really all about."

The event appears to have passed off without incident.

Race director Hugh Brasher said before the race that a full security review had taken place and everyone had been adamant that the "show must go on".

Image caption Many athletes wore black ribbons to remember Boston victims

The Metropolitan Police said it had reviewed security plans after the Boston attacks and that the extra officers would be used for "for reassurance patrols".

St John Ambulance said more than 5,000 runners and spectators received medical treatment at this year's event, slightly more than last year because of warmer weather during the early part of the race.

The first aid organisation's events manager Katherine Eaton said: "Most of the runners who needed treatment were suffering from cramp, sprains, exhaustion and vomiting.

"More than 40 competitors were taken to hospital suffering from more serious conditions including exercise-associated collapse and suspected fractures."

Australia's Kurt Fearnley was the first athlete to cross the line, winning the men's wheelchair race, with Great Britain's David Weir behind in fifth.

Tatyana McFadden won the women's wheelchair race less than a week after winning the equivalent race in Boston.

Media playback is unsupported on your device Media caption Guide to the security operation

She said: "You know this whole weekend was dedicated to Boston and we got huge support from London. So, I couldn't be happier - just getting support. It was just a wonderful day."

In the women's elite race, there was controversy early on when 2010 wheelchair winner Josh Cassidy dropped out after a collision with Olympic champion runner Tiki Gelana.

She continued running but was off the pace and her race was won by Kenyan Priscah Jeptoo in a time of 2:20:15.

The men's race was won by Ethiopian Tsegaye Kebede, who overtook 2011 champion Emmanuel Mutai in the closing stages, in a time of 2:06:04.

Mo Farah, who won the 5,000m and 10,000m at the 2012 London Olympics, ran only the first half of the 26.2-mile course.

Afterwards he praised the atmosphere of the crowd and said the experience was good practice ahead of him running the full race next year.

Singer Katherine Jenkins, cricketer Andrew Strauss and McFly's Harry Judd were among the well-known names running.

Among other runners taking part:

Former paratrooper Mike Ellicock broke a world record by running with a 40lb (18kg) pack in three hours 25 minutes and 21 seconds

A group of 15 people took part in memory of Leicestershire runner Claire Squires who died near the end of the course last year

Staff Sergeant Paul Chamberlain, of the Royal Corps of Signals, "completed" the course on a treadmill in Afghanistan, raising £1,000 for forces charities

Shadow chancellor Ed Balls thanked his political adversary George Osborne for sponsoring him as he completed his second London Marathon.

Prince Harry, who is the patron of the London Marathon Charitable Trust, gave the winners their medals.

Media playback is unsupported on your device Media caption Marathon runner hit by wheelchair

He said it was "never an option" that he would not be at the race because of security fears following the Boston bombings.

"The great thing about the marathon is no matter what colour you are, or religion, no matter what nationality you are, everyone comes together to run a certain distance to raise money for amazing causes.

"I think that you can never that take away from people."

Meanwhile, specialist US terrorism officers are waiting to question Dzhokhar Tsarnaev, the surviving suspect in the twin bombings close to the finish line of the Boston Marathon.

Mr Tsarnaev's older brother, Tamerlan, died after a shootout with police.

Virgin London Marathon has pledged to donate £2 for every runner that finishes Sunday's event to The One Fund Boston set up to raise money for victims of the explosions.