Two explosions shattered the finish of the Boston Marathon on Monday, bloodying spectators of America's oldest and most prestigious marathon. It was not immediately clear what closed the explosion or the number and extent of injuries.

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

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

"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.

The White House said President Barack Obama has been notified about the explosions. The administration said it is in contact with state and local authorities and the president directed his administration to provide whatever assistance is necessary in the investigation and response.

Vice President Joe Biden was on a conference call with gun control activists when staffers turned on televisions in his office Monday to view coverage of the explosions. Biden said during the call that his prayers were with those who suffered injuries.

About three hours after the winners crossed the finish 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.

Smoke rose from the blasts, fluttering through the national flags lining the route of the world's oldest and most prestigious marathon. TV helicopter footage showed blood staining the pavement in the popular shopping and tourist area known as the Back Bay.

"There are people who are really, really bloody," said Laura McLean, a runner from Toronto, who was in the medical tent being treated for dehydration when she was pulled out to make room for victims of the explosions. "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."

Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.

The nearby Prudential Tower, the city's second-tallest building with an upscale shopping mall on the ground, was evacuated, along with the luxury Mandarin Oriental hotel, according to media reports.

Race day got started with 26 seconds of silence in honor of the victims of the December school shooting in Connecticut. A little more than 2 hours later, the lead runners passed the Mile 26 marker, which was decorated with the Newtown, Connecticut, seal and dedicated to the memory of those killed there.

The annual 26.2-mile (42-kilometer) marathon takes place on Patriot's Day, a state holiday that celebrates the evacuation of Boston by the British in the American Revolution.