Police in Boston say three people have been killed, including an eight year old boy, after a double explosion at the city's marathon. 

More than 140 people have been injured with 8 of those believed to be in a critical condition - in what the FBI is calling a "potential terrorism investigation".

The news network CBS say police have surveillence video of people bringing multiple backpacks into the marathon area 20 minutes before the explosion.

The US Federal Aviation Administration has created a no-fly zone over the site of the explosions.

More than 23-thousand runners were due to take part in the marathon with over 100 Irish people registered to take part in the event.

So far all Irish runners have been accounted for.

Video from the scene shows a flash of flames and an enormous cloud of white smoke with about 20 seconds between the blasts.

A loud explosion went off on Boylston Street just before the bridge that marks the finish.

Another explosion could be heard a few seconds later reportedly on the other side of the street.

The explosions happened as the marathon was still taking place with some competitors pictured finishing the race and others being diverted away from the site.

Runners who had not finished made their way into a family meeting area, according to an emergency plan. 

The scene was described as chaotic  with emergency crews trying to rescue people trapped under fences and seating.

Meanwhile, a fire which broke out at the JFK library in the city, appears to have started in the mechanical room of new the building. It says all staff and visitors are accounted for and safe.

No one has been injured in that incident and it's now believed that the fire was a seperate incident and not related to the bomb blasts.

Boston Police Commissioner Edward Davis spoke lastnight about the fire:

US President Barack Obama addressed the nation following the incidents. He has ordered his administration to do everything necessary to investigate the explosions.

The Taoiseach last night sent his sympathy to the Governor, the Mayor and the people of Boston.

Enda Kenny said our thoughts and prayers are with the victims and their families.

The Department of Foreign Affairs have said this moring that all Irish have been accounted for but a special phone line has been set up for anyone looking to contact loved ones 01 478 0822.

Google have also launched a person finder for those looking for loved ones in the area.

TV helicopter footage showed blood on the pavement in the popular shopping and tourist area known as the Back Bay.

'There are people who are really bloody'

Runner Laura McLean of Toronto said she heard two explosions outside the medical tent.

"There are people who are really, really bloody," Ms McLean said. "They were pulling them into the medical tent."

As of yet it is not known what caused the explosions.

US President Barack Obama was informed of the blasts by the FBI Director 

In a conference call with reporters Vice President Joe Biden said "Our prayers are with those people in Boston who have suffered injuries."

State and local officials say there was no known credible threat prior to the blasts.

Meanwhile, in New York the Police Department says they have deployed units and evacuated landmarks as a response.

Corkman Bobby Hillard from Clonakilty was one of over 100 Irish runners who took part in the race.

He finished just 4 minutes before the explosion.