At least two people have been killed and 22 others injured after two explosions near the finish line of the Boston Marathon, police have said.
There was also a third blast at the JFK library about five miles away, officers confirmed.
The blasts at the marathon took place about three hours after the winners crossed the line, with one explosion on the north side of Boylston Street, just before the photo bridge that marks the finish.
Another happened a few seconds later, about 50-100 metres away, believed to be further down the street.
Some runners were making their way across the finish line as the drama unfolded, and TV helicopter footage showed blood on the pavement in the popular shopping and tourist area known as the Back Bay.
Participants were also seen lying on the ground as the explosions tore through the finish line, sending smoke and debris soaring into the air.
According to the Boston Marathon website, there were more than 25,000 registered entrants in the marathon, 374 of whom are British.
It is not known what caused the blasts but Fox News reported ball bearings were found on the street.
Sky's Mark White said: "The ball bearings are likely to be shrapnel, leading to a growing suspicion the blasts were deliberate."
There are also reports that police have discovered unexploded devices in the area.
Some runners who had not finished were diverted down Commonwealth Avenue and into a family meeting area, according to an emergency plan.
The Federal Aviation Administration has warned pilots it had created a no-fly zone over the site.
Runners and race volunteers were crying as they fled the chaos. Smoke rose from the explosions, through flags lining the route of the world's oldest and most prestigious marathon.
British police are now reviewing security plans for Sunday's London Marathon after the US blasts.
Video footage has emerged which appeared to show the moment one of the explosions happened.
Police made their way through competitors as they ran towards the scene, and injured spectators were being carried to the medical tent that had been set up to care for tired runners.
"There are a lot of people down," said one man, whose marathon number identified him as Frank Deruyter.
A Boston police officer was taken from the course with a bleeding leg injury.
Runner Laura McLean of Toronto said she heard two explosions outside the medical tent.
"There are people who are really, really bloody," Ms McLean said. "They were pulling them into the medical tent."
Cherie Falgoust was waiting for her husband, who was running the race.
"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."
Police have told people to stay at home and avoid large crowds.
The Marathon sports store in Boston was reportedly near where one of the blasts took place.
There were two boom sounds heard from inside the Fairmount Copley Plaza Hotel. Race officials locked the building down.
The White House said President Barack Obama has been notified about the explosions.
The administration said it is in contact with state and local authorities and the president told his administration to provide whatever assistance is necessary in the investigation and response.
New York police have stepped up security around key landmarks in the city after the double blasts, a top officer said.
Foreign Secretary William Hague tweeted: "Appalled by news of explosion at Boston marathon. My thoughts are with everyone affected by it and all those waiting for news."
Boston Police said there is a helpline in the US for concerned relatives : 617-635-4500, and anybody with information about the blasts should call 1-800-494-tips.
:: See Latest Updates On The Explosions