AMERICA is today coming to terms with one of the worst terrorist atrocities since 9/11  after three people people died and many more were injured when two bombs exploded close to the finish line of the Boston marathon.

There are unconfirmed reports that one of those killed is an eight-year-old boy.

The explosions came about four hours after the first runners crossed the finishing line and the area was still packed with spectators.

Boston police said three people died and dozens were injured. Medical authorities said they had treated more than 60 people.

President Barack Obama said it was still unknown who was responsible for the explosions or why.

But he pledged that he would find out who did it and said the responsible individuals or groups would feel the full weight of justice.

The US President told a news conference at the White House: "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.

"We salute all those who assisted in responding so quickly and professionally to this tragedy.

"We still do not know who did this, or why. And people shouldnt 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."

The first explosion occured around 3pm, local time, on Boylston Street just before the finish line. Seconds later another bomb detonated further down the street on the same side.

A third explosion, at the city's JFK library, is thought to be unrelated.

Police urged people to stay away from crowds and seek safety at home or in a hotel.

One of the devices is believed to have been hidden in a rubbish bin.

Ean Parsons, from Sedgefield Harriers, who was competing in the marathon spoke to The Northern Echo tonight to confirm that he was safe.

He finished the event  an hour before the explosions. His wife, Viv, is not competing but is also in Boston.

Mr Parsons told The Northern Echo: "I feel unbelievably lucky."

One unconfirmed report said an unexploded device has been on a glass footbridge over Huntington Avenue, near Copley Place. Another suspicious package has been reported near Harvard Station.

Fox News is reporting that many injuries were caused by ball bearings packed around an explosive device.

Massachusetts General Hospital has confirmed that it has taken four casualties from the scene.

FBI investigators with explosive sniffing dogs are on the scene as a massive security sweeps gets underway. There have also been a number of security scares at nearby hotels with guests evacuated onto the streets.

Reports are also coming in of other suspect packages being found in the city and American air force jets are now patrolling the skies.

Boston governor Deval Patrick said: “This is a horrific day in Boston. My thoughts and prayers are with those who have been injured. I have been in touch with the President, Mayor Menino and our public safety leaders. Our focus is on making sure that the area around Copley Square is safe and secured. I am asking everyone to stay away from Copley Square and let the first responders do their jobs.”

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

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

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.

Prime Minister David Cameron wrote on Twitter: "The scenes from Boston are shocking and horrific - my thoughts are with all those who have been affected."

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."

In response to the Boston explosion, New York has tonight upgraded its anti-terrorism stance - putting extra security in place around major hotels and public buildings.

Paul Browne, deputy police commissioner of the NYPD, said police were deploying counter-terrorism vehicles.

Security has also been stepped up in Washington DC around the White House.

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."

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