Explosion at the Boston Marathon

According to Sky News at least two people have been killed and 22 others injured after two explosions at the finish line of the Boston Marathon, police have said.

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.

Another explosion could be heard a few seconds later, reportedly on the other side of the street.

TV helicopter footage showed blood on the pavement in the popular shopping and tourist area known as the Back Bay.

Fox News reports that ball bearings have been found on the street.

Sky's Mark White said: "The ball bearings are likely to be shrapnel, leading to a growing suspicion the blasts were deliberate."

 At least two people have been killed and 22 others injured after two explosions at the finish line of the Boston Marathon, police have said.

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.

Another explosion could be heard a few seconds later, reportedly on the other side of the street.

TV helicopter footage showed blood on the pavement in the popular shopping and tourist area known as the Back Bay.

Fox News reports that ball bearings have been found on the street.

Sky's Mark White said: "The ball bearings are likely to be shrapnel, leading to a growing suspicion the blasts were deliberate."

Smoke rose from the explosions, through flags lining the route of the world's oldest and most prestigious marathon.

The Marathon sports store was reportedly near where one of the blasts took place.

There were two boom sounds heard from inside the Fairmount Copley Plaza Hotel.

Race officials have locked down the hotel.

A photo has emerged which appeared to show the moment one of the blasts took place.

Police made their way through competitors as they ran towards the scene.

Bloody spectators were being carried to the medical tent that had been set up to care for tired runners.

"There are a lot of people down," said one man, whose marathon number identified him as Frank Deruyter, who was not injured.

 But marathon workers were carrying one woman, who did not appear to be a runner, to the medical area whose leg was hurt.

A Boston police officer was taken from the course with a bleeding leg injury.

Runner Laura McLean of Toronto said she heard two explosions outside the medical tent.

"There are people who are really, really bloody," Ms McLean said. "They were pulling them into the medical tent."

Cherie Falgoust was waiting for her husband, who was running the race.

"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."

Foreign Secretary William Hague tweeted: "Appalled by news of explosion at Boston marathon. My thoughts are with everyone affected by it and all those waiting for news."