Police officials say mobile phone service have been shut down in the Boston area to prevent any potential remote detonations of explosives.
Authorities said at two bombs simultaneously struck the Boston Marathon as runners crossed the finish line just before 3pm on Monday. They said two people were dead and at least 23 injured, with six critically wounded in the blasts.
The Boston Globe reported that more than 100 people had been treated at hospitals.
A third incident occurred at the John F. Kennedy Library several miles away more than an hour later, but no injuries were reported. There were conflicting reports as to whether there had been an explosion or a mechanical fire at the library.
A senior US intelligence official said two more explosive devices had been found near the scene of the explosions and were being dismantled. Separately, the Federal Aviation Administration issued a no-fly zone over the site.
A Rhode Island state police officer, who was running in the event, said he saw at least two dozen people with very serious injuries, including missing limbs.
Injured spectators were being carried to the medical tent that had been set up to care for fatigued runners.
The stragglers in the 40km trek were rerouted away from the smoking site of the blasts, as a Massachusetts General Hospital spokesperson said it had four patients from the incident under evaluation and was expecting more.
White House cordon
A Washington police spokesman told Reuters news agency the area in front of the White House was cordoned off with tape after news of the explosions broke.
The White House said President Barack Obama had been notified about the incident and directed his administration to provide whatever assistance necessary for the investigation.
The explosion occurred about three hours after the winners crossed the line as thousands of runners finished the 117th running of the Boston Marathon, with crowds watching and cheering at the finish.
Competitors and race organisers were crying as they fled the chaos . Bloody spectators were being carried to the medical tent that had been set up to care for fatigued runners.
"There are a lot of people down,'' said one man, whose bib no 17528 identified him as Frank Deruyter.
He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg.
A Boston police officer was wheeled from the course with a leg injury that was bleeding.
Neither race officials nor public officials could immediately estimate the number or degree of injuries, but the Boston Globe newspaper reported that at least 12 people were injured.
Laura McLean, a runner from Toronto, Canada, said she heard two explosions outside the medical tent.
"There are people who are really, really bloody,'' McLean said. "They were pulling them into the medical tent.''
Loud explosion
About three hours after the winners crossed the line, there was a loud explosion on the north side of Boston's Boylston Street, just before the photo bridge that marks the finish line.
Another explosion could be heard a few 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.
Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.
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