Two bombs exploded near the crowded finish line of the Boston Marathon on Monday, killing two people and injuring more than 130 in a terrifying scene of broken glass, smoke and severed limbs at the world's oldest and most prestigious race, authorities said.

A senior US intelligence official said two other explosive devices were found near the marathon finish line.

There was no word on the motive or who may have launched the attack, and authorities in Washington said there was no immediate claim of responsibility.

A victim is attended to at the scene of an explosion at the Boston Marathon in Boston, Massachusetts, April 15, 2013. [Photo/Agencies]

The twin blasts at the race took place almost simultaneously and about 100 yards (meters) apart, tearing limbs off numerous people, knocking spectators and at least one runner off their feet, shattering windows and sending smoke rising over the street.

As people wailed in agony, bloody spectators were carried to a medical tent that had been set up to care for fatigued runners.

"They just started bringing people in in with no limbs," said Tim Davey, of Virginia. He said he and his wife, Lisa, tried to keep their children's eyes shielded from the gruesome scene.

Some 27,000 runners took part in the 26.2-mile (42-kilometer) race, one of the world's premier marathons and one of Boston's biggest annual events.

After the explosions, mobile phone service was shut down in the area to prevent any possible remote explosive detonations, a law enforcement official said. The official spoke on condition of anonymity because the investigation was ongoing.

Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads checked parcels and bags left along the race route.

A third blast rocked the John F. Kennedy Library a few miles (kilometers) away and more than an hour later, but no injuries were reported, the police commissioner said.

The Federal Aviation Administration barred low-flying aircraft from within 3.5 miles (5.6 kilometers) of the site.

President Barack Obama was briefed on the explosions by Homeland Security adviser Lisa Monaco. Obama also told Boston Mayor Tom Menino and Gov. Deval Patrick that his administration would provide whatever support was needed, the White House said.

Obama planned to issue a statement Monday evening in televised remarks from the White House.

"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.