Two people have been killed and 23 injured after two explosions at the finish line of the Boston Marathon, police say.
The blasts shattered the carnival atmosphere of one of the biggest sporting events in the US, causing panic and confusion.
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.
The cause of the explosions has yet to be confirmed.
It was a beautiful day in Boston,
and everyone was in high spirits
for the marathon.
I was in Washington Square,
about two miles from the finish
line, where a band was playing
and there was a carnival atmosphere.
Suddenly the news started to filter
through that there had been an
explosion near the finish line.
The music turned off and the mood
changed quickly. People reached for
their mobile phones to reassure loved
ones, and the crowds dissipated fast.
Small groups of runners and their
supporters huddled together, pooling
information and trying to find out just
what had happened in Boston on
A spokeswoman for Massachusetts General Hospital in the city told Reuters news agency that it was treating 19 victims of the explosion in its emergency room, but information about their condition was not immediately available.
A White House official said President Barack Obama had been notified about events in Boston, 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."
President Obama had called Boston Mayor Tom Menino and Massachusetts Governor Deval Patrick on Monday to offer federal assistance, an official said.
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.
The incident reportedly came about two hours after the winners crossed the line.
The Associated Press news agency said there was a loud explosion on the north side of Boylston Street, just before the bridge that marks the finish line. Another loud explosion could be heard a few seconds later, and smoke could be seen rising from the scene of the blasts.
TV helicopter footage showed blood staining the pavement in the shopping and tourist area known as Back Bay.
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.
Stragglers heading for the finish line were rerouted away from the smoking site of the blasts.
The annual Boston Marathon attracting a large field of runners and tens of thousands of spectators.
British police are reviewing security plans for Sunday's London Marathon, the next major international marathon, following events in Boston.