A runner's camera captured the first explosion at the finishing line of the Boston Marathon. Three people were killed and more than 100 injured as two blasts sent smoke and debris soaring into the air.
Boston Police Commissioner Edward Davis has told reporters there were three separate explosions in Boston - the first two near the finish line of the marathon and a third at the John F. Kennedy Library in the city.
He said that police were treating the explosions as related.
Here is an excerpt from the briefing he just gave to reporters:
The JFK Library in Boston has tweeted to say that there was fire in the building which has now been extinguished and that it is "pure speculation" to link it with the Boston Marathon explosions:
Fire in building is out, appears to have started in the mechanical room of new building. All staff and visitors are accounted for and safe.
Investigators are investigating. Any tie to Boston Marathon explosions is pure speculation. More information as we receive it.
Boston police have confirmed to Reuters that they have carried out another controlled explosion outside the JFK Library in Boston which is in another part of the city from the finish line of the Marathon.
Paul Browne, the New York Police Department commissioner, has told Reuters New York will be stepping up security in response to the explosions at the Boston Marathon.
Browne added the police department would be deploying 'counter-terrorism vehicles' around landmarks in Manhattan, including 'prominent hotels'.
David Ryan, a photographer for local newspaper the Boston Globe has tweeted a picture of the moment an explosion rocked the finish line at the Boston Marathon:
boston marathon explosion http://t.co/xUZ4EXKhLZ
The windows of the buildings surrounding the finish line of the Boston Marathon were broken and debris was scattered across the ground after a reported explosion.