Two explosions at the finish line of the Boston Marathon race have left an unknown number of people injured.
Video and photographs from Boston show a scene of confusion, with emergency services descending on the scene and bloodied spectators being taken to a medical tent.
"There are a lot of people down," said one runner quoted by AP news agency.
The incident reportedly came about three hours after the winners crossed the line.
AP said there was a loud explosion on the north side of Boylston Street, just before the bridge that marks the finish line. Another loud explosion could be heard a few seconds later.
Stragglers heading for the finish line were rerouted away from the smoking site of the blasts.
The Eastern Massachusetts branch of the Red Cross has set up a disaster response centre in the area.
Police say two explosions at the finish line of the Boston Marathon have resulted in injuries.
Bloody spectators were being carried on Monday to the medical tent that had been set up to care for fatigued runners.
Police wove through competitors as they ran back towards the course.
"There are a lot of people down,'' said one man, whose bib No 17528 identified him as Frank Deruyter of North Carolina.
He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg.
A Boston police officer was wheeled from the course with a leg injury that was bleeding.
About three hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line.
Another explosion could be heard a few seconds later.
See more photos from the bomb blast below:
