Britons are feared among the injured after two explosions rocked the Boston Marathon killing two and injuring dozens of people.
According to the Boston Marathon website, there are more than 25,000 registered entrants, 374 of whom are British.
Horrifying images of injured spectators and blood-splattered pavements were caught as television cameras broadcast footage of the blasts.
A Foreign Office spokesman said: "We are aware of the incident and we are trying to gather information."
A British runner has told of how he and his family narrowly avoided the explosions in Boston after he completed the marathon.
Darren Foy, 40, from Southampton and his wife Sandra and their two children missed the blast by just 30 minutes after he finished the race in three and half hours.
Speaking from the city, the chartered surveyor said: "We were on our way home when we heard something had happened and I was getting messages like 'are you Ok?' which I never get. It's all quite shocking really.
"We got home and we looked at the BBC online and saw there had been explosions but we are OK because I finished in three and a half hours and we were on the bus when it happened.
"We are staying five miles outside and I'm not intending to go back into the city for a few days."
Mr Foy, who is chairman of the Lordshill Road Runners in Southampton and was competing in his fourth marathon, added: "There are reports here that the explosions came from a hotel at the finish line and I walked past there a few days ago to pick up my race number.
"It's such a soft target. There are hundreds of thousands spectators on the streets and 27,000 runners, so we got off lightly."
UK Athletics said that none of their elite runners had taken part and added that Ross Murray, who ran yesterday's one mile invitational was "absolutely fine".
Twenty-two-year-old Murray Tweeted: "Just so everyone knows, I'm ok and well, wasn't around the bombs. Xx"
"In the hotel, most seem ok but a few seem shaken up, witnessed a few very relieved families meeting up."
[How the news broke on Twitter]
Tatyana McFadden, who won three golds in the London Paralympics, won the women's wheelchair race and later Tweeted: "I'm am okay my family and teammates are okay. Please please pray for those who were running and got hit by the bomb. Pray 4 family's-friends."
Nick Bitel, the chief executive of London Marathon, 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."