Britons were tonight feared to be among those injured after two explosions ripped through the finish line of the Boston Marathon.
According to the Boston Marathon website 374 British and 108 Irish runners were due to take part in the race.
If you are concerned about a friend or relative the site (http://raceday.baa.org/individual.html) may provide some details.
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.
Pictures also emerged on Twitter showing casualties lying on the pavement on Boylston Street - the main road through the east coast city - and debris blowing around them.
Another photograph appeared to show participants running down the street at the moment an explosion created a fireball, sending smoke into the air.
A list of competitors on the Boston Marathon website showed hundreds of British runners were expected to line up for the race.
A Foreign Office spokesman said: "We are aware of the incident and we are trying to gather information."