Some 347 of the 25,000 taking part in the race were British and several told of the aftermath of the two blasts.
Mike Kendall, from Northampton, tweeted after completing the 26.2 mile race:
My thoughts are with @bostonmarathon after explosions, 2 dead and 22 injuries after such a good day with volunteers, runners and crowds
Anthony Meenaghan, 21, an architectural technology student from Sheffield Hallam University who is on a work placement in Boston, tweeted
Thanks for all messages. I'm safe and well. Can't believe what I saw and heard. Sad day
More top news
Heartwarming image of elementary school teacher walking pupils home prompts interest in a scheme that has been running for three years.
The PM used his speech at the Tory party conference to take a swipe at Labour rival Jeremy Corbyn but what else did he have to say.
Pc David Phillips' widow broke down in tears as his sisters and best friend paid tribute to the "man they adored".