At least three people have been killed and more than 100 injured, some seriously, in two explosions at the finish line of the Boston Marathon.
TV footage showed bloodied runners and spectators being treated at the scene and the road strewn with debris.
The FBI said that this was a "potential terrorist investigation".
In a TV address, President Barack Obama said "we will find out who did this" and that 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.
At an initial news conference, Boston Police Commissioner Edward Davis said authorities had received "no specific intelligence that anything was going to happen".
He urged people to stay indoors and not congregate in large groups.
At a second conference he said the death toll now stood at three. He said that no suspects were in custody.
Governor Patrick, speaking at the same news conference, confirmed reports that more than 100 people had been injured, some gravely.
He said Boston would be "open" on Tuesday but that there would be "a heightened law enforcement presence".
"There will be random checks of backpacks and other parcels. We are also asking that everyone be on a state of heightened vigilance," he said.
Details of the victims have not been revealed, however the Associated Press news agency reported that an eight-year-old boy was among the dead. Quoting a family friend, the report said the boy's mother and sister were also injured as they waited for his father to finish the race.
The city mayor's office has set up an emergency hotline for friends and relatives on +1 617 635 4500.
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."
A doctor at Massachusetts General Hospital said "several amputations" had been performed there.
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 sending some runners tumbling to the pavement while others were knocked down by those rushing to the scene.
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 might have been caused by an incendiary device but it is unclear whether it is related to the bombings.
The annual Boston Marathon this year had a field of about 23,000 runners and was watched by hundreds of thousands of spectators.
It is held on Patriots Day, a Massachusetts state holiday which commemorates the first battles of the American Revolution in 1775.
British police are reviewing security plans for Sunday's London Marathon, the next major international marathon, following events in Boston.