Two people have been killed and at least 23 injured in two explosions at the finish line of the Boston Marathon, police say.
TV footage showed bloodied runners and spectators being treated at the scene and the road strewn with debris.
Boston police said a third incident occurred at the city's JFK Library and was under investigation.
In a TV address, President Barack Obama said those responsible would feel the "full weight of justice".
"We don't yet have all the answers," he said. "We still do not know who did this or why."
President Obama said he had called Boston Mayor Tom Menino and Massachusetts Governor Deval Patrick to offer federal assistance.
He said the government would increase security around the US "as necessary'' but did not say whether the White House thought the incident was part of a larger plot.
Mr Obama stopped short of calling the incident an act of terrorism.
The BBC's North America editor Mark Mardell says perhaps this was merely caution, although given that the president will face criticism for not using that word, it is certainly deliberate and may be significant.
Boston's police commissioner has urged people in the city to stay indoors, and not congregate in large groups.
The city mayor's office has set up an emergency hotline for friends and relatives on +1 617 635 4500.
A spokeswoman for Massachusetts General Hospital in the city told Reuters news agency that it was treating 19 victims of the explosions in its emergency room, but information about their condition was not immediately available.
The Federal Aviation Administration has created a no-fly zone over the area, while security at key sites in Washington DC and New York has been tightened.
Vice President Joe Biden - breaking off from a telephone conference call on gun control - said: "Our prayers are with those people in Boston who have suffered injury."
State police officer Roupen Bastajian had just finished the race when he heard the blasts.
"I started running toward the blast and there were people all over the floor," he said.
"We started grabbing tourniquets and started tying legs. At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."
The first explosion came at about 14:50 local time (18:50 GMT), approximately two hours after the winners crossed the line.
There was a loud explosion on the north side of Boylston Street, just before the finish line. Another loud explosion occurred a few seconds later, and smoke rose from the scene of the blasts.
Bloodied victims were initially rushed to a medical tent set up to care for fatigued runners.
Emergency services descended on the scene, which was quickly locked down.
Stragglers heading for the finish line were rerouted away from the smoking site of the blasts.
Mike Mitchell of Vancouver, Canada, a runner who had finished the race, said he was looking back at the finish line when he saw a "massive explosion."
Smoke rose 50ft (15m) in the air, he told Reuters news agency, and people began running away and screaming after hearing the noise. "Everybody freaked out," he said.
A fire then broke out at the John F Kennedy presidential library a few miles away from the finish line.
Police said it may have been caused by an incendiary device but did not appear to be related to the bombings.
The annual Boston Marathon attracts a large field of runners and hundreds of thousands of spectators.
British police are reviewing security plans for Sunday's London Marathon, the next major international marathon, following events in Boston.
President Obama: "We will find out who did this and hold them accountable"