The first 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.
The 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'.