Two bombs have exploded near the crowded finish line of the Boston Marathon, killing three people and injuring more than 130 others.
A senior US intelligence official said two other explosive devices were found near the marathon finish line.
There was no word on the motive or who may have launched the attack, and authorities in Washington said there was no immediate claim of responsibility.
US President Barack Obama said in televised remarks that the US does not know "who did this or why", but vowed that whoever is responsible "will feel the full weight of justice."
He said: "We will find out who did this and we will hold them accountable."
Mr Obama made his remarks from the White House about three hours after the explosions.
The twin blasts at the race took place almost simultaneously and about 100 metres apart, tearing limbs off numerous people, knocking spectators and at least one runner off their feet, shattering windows and sending smoke rising over the street.
As people wailed in agony, bloody spectators were carried to a medical tent that had been set up to care for fatigued runners.
Some 27,000 runners took part in the 42-km race, one of the world's premier marathons and one of Boston's biggest annual events.
After the explosions, mobile phone service was shut down in the area to prevent any possible remote explosive detonations, a law enforcement official said. The official spoke on condition of anonymity because the investigation was ongoing.
Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads checked parcels and bags left along the race route.
The Department of Foreign Affairs said the Irish Consulate in Boston is in touch with the city's authorities with a view to establishing whether any Irish people have been injured in today's explosion at the Boston marathon.
According to the official Boston Marathon website, 50 competitors listed Ireland as their country of residence, and 108 have listed Ireland as their country of citizenship.
The Department is advising anyone who is concerned to contact them on 01 478 0822.
The US Federal Aviation Administration barred low-flying aircraft from within 5.6km of the site.
About two hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another explosion could be heard a few seconds later.
A senior US intelligence official said the two other explosive devices found nearby were being dismantled. The official spoke on condition of anonymity because he was not authorised to discuss the findings publicly.
Smoke rose from the blasts, fluttering through the national flags lining the route in the heart of central Boston.
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.
Vice President Joe Biden was on a conference call with gun control activists when staffers turned on televisions in his office to view coverage of the explosions.
"Apparently there has been a bombing," Mr Biden said.
"I don't know any of the details of what caused it, who did it. I don't think it exists yet. But our prayers are with those people in Boston who suffered injury."
Shortly after the explosions, Secret Service shut down Pennsylvania Avenue outside the White House, cordoning off the area with yellow police tape.
Several Secret Service patrol cars also blocked off the entry points to the road.
The White House was not on lockdown and tourists and other onlookers were still able to be in the park across the street from the executive mansion.
Attorney General Eric Holder directed the full resources of the Justice Department be deployed to investigate and a department official said Holder has spoken with FBI Director Robert Mueller.
The official said the US attorney for Massachusetts's office was coordinating the Justice Department's response with the FBI and other federal, state and local law enforcement authorities.
Cities worldwide stepped up security following the explosions.
In Britain, police said they were reviewing security plans for Sunday's London Marathon, the next major international marathon. Thousands of people compete in the London Marathon every year, thronging the city's streets.
In New York City, police spokesman Paul Browne said that critical response teams are deployed around the city.
The annual marathon attracts more than 500,000 spectators. It takes place on Patriot's Day, a state holiday that celebrates the evacuation of Boston by the British in the American Revolution.
President, Taoiseach send sympathies
President Michael D Higgins sent his sympathy to the Governor, Mayor and the people of Boston:
"I am deeply sorry to learn of the explosions in Boston earlier today that have resulted in fatalities and injuries.
“The occurrence of the explosions during the course of the Boston Marathon is deeply shocking and is a very sad end to what should have been a day of great joy and achievement for the thousands, including many Irish citizens, who participate in and watch this widely supported annual event.”
Taoiseach Enda Kenny has also sent his sympathies.
"On behalf of the Irish people, I am sending our sympathy and support to Mayor Menino and Governor Patrick and to the people of Boston who have been devastated by this senseless and terrible event.
“Our thoughts and prayers are with the victims and their families."