As it happened.
Two powerful explosions rocked the finish line area of the Boston Marathon near Copley Square. The Boston Police Department suggests 3 are dead, and at least 140 are thought to be injured.
A third explosion later occurred at the JFK Presidential Library.
CBS News has reported a police source has said that they believe the explosives were planted in trash cans. The same source also told CBS News they believe they have surveillance video of a possible suspect - a man carrying multiple backpacks.
'There are a lot of people down', one runner was quoted as saying by AP news agency.
An intelligence official has said two more explosive devices found near the site are being dismantled.
'Bags and parcels' left behind by evacuating runners are all being treated as 'possible incendiary devices', Boston police say.
A White House official said President Barack Obama had been notified about events, while Vice President Joe Biden - breaking of from a telephone conference call on gun control - said: 'Our prayers are with those people in Boston who have suffered injury'.
The first explosion caused a huge puff of white smoke and was followed by a smaller blast. One man was covered in shattered glass, with bandages around his head. At least dozens of people have been seriously injured, the Boston Globe reported on its Twitter feed.
Associated Press reported that the authorities were helping injured runners leave the scene and bloody spectators were being carried to a medical tent that was being used for runners.
The A.P. said that a loud explosion was heard on the north side of Boylston Street, near a photo bridge that marks the finish line. Another explosion was heard several seconds later.
Smoke rose from the blasts, fluttering through the national flags lining the route of the world's oldest and most prestigious marathon. TV helicopter footage showed blood staining the pavement in the popular shopping and tourist area known as the Back Bay.
'There was an explosion, police, fire and EMS are on the scene. We have no indication of how many people are injured', spokeswoman for the Boston Police Department said.
Mike Mitchell of Vancouver, Canada, a runner who had finished the race said he was looking back at the finish line and saw a 'massive explosion'.
President Barack Obama was notified, and his administration was in touch with state and local authorities, a White House official told NBC News. Obama directed the administration to provide whatever help was necessary, the official said.
Within the first hour after the blasts, New York police deployed counterterrorism vehicles to landmarks, and Pennsylvania Avenue in front of the White House was closed to foot traffic.
New York City police are stepping up security at hotels and other prominent locations following explosions at the Boston Marathon.
Chief NYPD spokesman Paul Browne said critical response teams are deployed around the city until more about the explosion is learned.
As the situation is both breaking and dynamic, details about the background of the bombing are slowly emerging. An analysis of open-source intel, currently available, suggests that there is indeed a potential for there to be involvement on behalf of a foreign terrorist organization or even a home grown terrorist cell. As of now, Boston Police are operating under the assumption that this is a 'possible terrorist attack'.
No-one has yet claimed responsibility for the acts.
A law enforcement official says cellphone service has been shut down in the Boston area to prevent any potential remote detonations of explosives.
With little official information to guide them, members of Congress said Monday there was scant or no doubt that the deadly Boston Marathon explosions were acts of terrorism and vowed to bring anyone responsible to justice.
'We just don’t know whether it’s foreign or domestic', said Rep. Michael McCaul, R-Texas, chairman of the House Committee on Homeland Security.
'My understanding is that it’s a terrorist incident', Sen. Dianne Feinstein told reporters.
British police say they are reviewing security plans for Sunday's London Marathon, the next major international marathon, because of the explosions that hit the race in Boston.