Photographs from Boston show a scene of confusion, with emergency services descending on the scene and bloodied spectators being taken to a medical tent.
The world oldest marathon has seen two explosions at the finish line, the second explosion coming just seconds after the first.
The First explosion was reported to be at Boylston Street, a short way from the bridge that marks the finish line.
TV Helicoptors showed filming the event showed blood stained pavements in the poular shopping area known as Back Bay.
The Boston police department say they do not know yet how many people have been injured but reports say at least 2 people have died with 22 injured.
The Boston Marathon is one of the biggest sporting events in the United States attracting a huge fiedls of runners and many thousands of spectators.
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