Britons are feared to be among those injured after two explosions ripped through the finish line of the Boston Marathon.
Television pictures showed bloodied spectators and runners lying on the ground, emergency workers ripping away fencing and others carrying injured men and women from the scene.
A list of competitors on the Boston Marathon website showed hundreds of British runners were expected to line up for the race.
The site lists more than 25,000 participants, 347 of which are British although it is unclear whether any of them, or their supporters watching the race, were caught in the blasts.
A Foreign Office spokesman said: “We are aware of the incident and we are trying to gather information.”
The suspected terror attack has inevitably raised questions about this weekend’s London Marathon, due to be held on Sunday.
Race chief Nick Bitel said: “We are deeply saddened and shocked by the news from Boston. Our immediate thoughts are with the people there and their families. It is a very sad day for athletics and for our friends and colleagues in marathon running.
“Our security plan is developed jointly with the Metropolitan Police and we were in contact with them as soon as we heard the news.”
Met Police Chief Superintendent Julia Pendry said: “A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon.”