Some 347 Britons and 108 Irish runners took part in the Boston race, which was hit by two explosions yesterday evening.
The Foreign Office said it is "not aware" of any casualties of British nationals but the consulate in Boston is advising people to contact a hotline for relatives if they are worried.
"There have been no reports of any British national casualties that we are aware of," a spokesman said. "We have no reason to believe that any British people were caught up in it."
"We would advise all non-U.S residents who are out there to follow the advice of local authorities in Boston. Some medical tents were set up yesterday and people were advised to stay at home. We would always say people should follow the advice of local authorities."
+1-617-635-4500 is the number to call if you had a family member or friend at the scene of the Boston marathon explosion. Everyone at FirstPoint USA sends their thoughts to the great city of Boston and all those affected by the tragic events.
