AT least two people have died and at least 23 others have been injured after three explosions near the finish line of the Boston Marathon.
The US city was a picture of confusion and panic as emergency services descended on the scene and bloodied spectators were carried to a medical tent set up to care for fatigued runners.
Scores of Scots were among the 374 UK runners registered as taking part in the event, which had about 27,000 competitors.
Unconfirmed reports suggested up to 12 people had been killed, with dozens seriously hurt as the site of the world's oldest marathon was locked down.
Police are understood to have been finding "secondary devices" were pleading with anyone in the area to leave. In particular, authorities urged people to stay away from rubbish bins.
The blasts occurred at about 2.45pm local time, about two hours after the first of the runners had crossed the finish line to the noise of cheering spectators.
As competitors and race organisers were seen crying while fleeing the chaos, troops from the Massachusetts National Guard were called in to assist in the streets, which were stained with blood.
One spectator, Dave Weigel, wrote on Twitter minutes after the explosion: "I saw people's legs blown off. Horrific. Two explosions. Runners were coming in and saw unspeakable horror."
Witnesses said both blasts occurred within 30 seconds of each other and appeared to have come from the pavement over 100 yards from the finish line at Copley Square in downtown Boston, on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another blast went off outside the JFK Library in William T Morrissey Boulevard.
Windows of shops were blown out by the force of the blasts as the area around Copley Square was turned into a scene of destruction.
The Secret Service shut down Pennsylvania Avenue outside the White House in Washington DC, cordoning off the area with yellow police tape. Several Secret Service patrol cars also blocked off the entry points to the road.
Mike Mitchell, of Vancouver, Canada, a runner who had finished the race, said he was looking back at the finish line and saw a "massive explosion".
Smoke rose 50ft in the air, and people began running and screaming after hearing the noise, Mr Mitchell said. "Everybody freaked out," he added.
Stragglers heading for the finish line were rerouted away from the smoking site of the blasts as the area was secured.
The Eastern Massachusetts branch of the Red Cross has set up a disaster response centre.
Officials organising this Sunday's London Marathon are now reviewing security arrangements in partnership with the Metropolitan Police in the wake of the Boston blasts.
Nick Bitel, London Marathon chief executive, 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."
President Barack Obama spoke to Boston mayor Tom Menino and Massachusetts governor Deval Patrick in the wake of the explosions. Mr Obama expressed his concern for those who were injured and said his administration was ready to provide support.
The White House said in a statement: "The President directed his administration to provide whatever assistance is necessary in the investigation and response."
Onlooker Josh Matthews said he heard the blast, then saw police running toward the scene.
"We just heard a lot of sirens, and people were kind of frantic, and it was a bad situation, so we got out of there," he said.
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