Death and destruction descended on the Boston Marathon yesterday as two blasts rocked the finish line, killing three and injuring more than 100 people.

Horrifying images of injured spectators and blood-splattered pavements were caught as television cameras broadcast footage of the explosions.

Participants were also seen lying on the ground as the impact tore through the finish line, sending smoke and debris soaring into the air.

Emergency workers ripped away fencing as others carried injured men and women from the scene.

A list of competitors on the Boston Marathon website showed hundreds of British runners were expected to line up for the race.

The site lists more than 25,000 participants, 347 of which are British - although it is unclear whether any were caught in the blasts.

The Boston marathon - one of the world's oldest - is held each year on Patriot's Day.

Ed Davis, the city's Police Commissioner, said there had been two "simultaneous explosions" near the finish line of the 26.2 mile event around four hours into the race.

"Each scene resulted in multiple casualties," he told a press conference.

Commissioner Davis said there had also been a third explosion at the JFK Library in the city, but he did not believe there were any casualties as a result of that blast.

He said: "We are not certain that these incidents are related, but we are treating them as if they are.

"We are recommending to people that they stay at home, that if they're in hotels in the area that they return to their rooms and that they don't go to any place to congregate in large crowds.

"We want to make sure we completely stabilise the situation."

He said that the FBI (Federal Bureau of Investigation), State Police and National Guard were working with other agencies in the aftermath.

Bomb disposal teams carried out a controlled explosion on another suspected device, and Commissioner Davis said a number of unattended packages along the marathon route were being treated with caution.

British runners described how they avoided being caught up in the deadly explosions by a matter of minutes.

Runner Darren Foy, 40, from Southampton, and his wife Sandra and their two children, missed the explosions by just half an hour after he finished the marathon in three and a half hours.

Speaking from the city, the chartered surveyor said: "We were on our way home when we heard something had happened and I was getting messages like 'are you Ok?' which I never get. It's all quite shocking really.

"We got home and we looked at the BBC online and saw there had been explosions but we are OK because I finished in three and a half hours and we were on the bus when it happened.

"We are staying five miles outside and I'm not intending to go back into the city for a few days."

Mr Foy, who is chairman of the Lordshill Road Runners in Southampton and was competing in his fourth marathon, added: "There are reports here that the explosions came from a hotel at the finish line and I walked past there a few days ago to pick up my race number.

"It's such a soft target. There are hundreds of thousands spectators on the streets and 27,000 runners, so we got off lightly."

Jez Hughes, a firefighter from Morley, west Yorkshire, completed the marathon - his 10th - and was walking to the subway with his wife when they heard two explosions.

He had been standing only 100 yards from the finish line with other competitors but walked around the corner to meet his wife.

"I had already finished and a few of us had congregated 100 yards past the finish line," he said.

"I was waiting for my wife as I had asked her to meet me there but she was in the family area. Fortunately for me I went round the corner there to meet her.

"While we were going to the subway we heard two explosions. I said straight away that sounds like a bomb and then we heard a second one.

"We were a couple of blocks out of the way so it did not affect us.

"My wife is very shook up but we are out here until Thursday evening and no terrorists are going to spoil our day, don't let them get the better of you.

"Our condolences go out to those who have lost loved ones."

Since returning to his hotel, Mr Hughes, who was running for The Fire Fighters Charity, said he has been able to contact his two teenage children and reassure them that they are safe.

Politicians took to Twitter to comment on the incident.

Foreign Secretary William Hague tweeted: "Appalled by news of explosion at Boston marathon. My thoughts are with everyone affected by it and all those waiting for news."

The Labour leader Ed Miliband wrote: "Appalling news from Boston. All of my thoughts are with the casualties and their families."

London Mayor Boris Johnson tweeted: "Shocked by events at the Boston Marathon - my thoughts and prayers are with the victims and their families tonight."

The blasts come just six days before the London Marathon on Sunday, prompting a "review" of security ahead of the event.

Chief Superintendent Julia Pendry, of the Met Police, the event commander for the race, said security arrangements will be reviewed following the explosions in Massachusetts.

She said: "A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon."

Nick Bitel, the chief executive of London Marathon, said: "We are deeply saddened and shocked by the news from Boston.

"Our immediate thoughts are with the people there and their families. It is a very sad day for athletics and for our friends and colleagues in marathon running.

"Our security plan is developed jointly with the Metropolitan Police, and we were in contact with them as soon as we heard the news."

A Foreign Office spokesman said: "We are aware of the incident and we are trying to gather information."

In a televised address, US President Barack Obama said those responsible will feel the "full weight of justice".

Speaking from the White House, he paid tribute to those who helped wounded runners and spectators, saying: "The American people will say a prayer for Boston tonight, and Michelle and I send our deepest thoughts and prayers to the families of the victims in the wake of this senseless loss.

"We don't yet have all the answers but we do know that multiple people have been wounded, some gravely, in explosions at the Boston Marathon."

Mr Obama added: "We still do not know who did this, or why. And people shouldn't jump to conclusions before we have all the facts.

"But, make no mistake, we will get to the bottom of this, and we will find out who did this, we will find out why they did this.

"Any responsible individuals, any responsible groups, will feel the full weight of justice."