Explosions rock Boston Marathon, 2 killed and several injured (Video and Photos)

Two explosions at the finish line of the Boston Marathon race have left 2 people dead and an unknown number of 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.

"There was an explosion, police, fire and EMS are on the scene. We have no indication of how many people are injured," spokeswoman for the Boston Police Department said.

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.

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 (15m) in the air, he told Reuters news agency, and people began running away and screaming after hearing the noise.

"Everybody freaked out," he said.

Stragglers heading for the finish line were rerouted away from the smoking site of the blasts as the scene was locked down.

The Eastern Massachusetts branch of the Red Cross has set up a disaster response centre in the area.