BOSTON — Two bombs exploded near the finish line of the Boston Marathon on Monday, killing two people and injuring at least 23 others in a terrifying scene of broken glass, smoke and severed limbs, authorities said.

There was no immediate word on the motive or who may have launched the attack and authorities in Washington said there was no immediate claim of responsibility. Some 27,000 runners took part in the 26.2-mile race, one of the world’s premier marathons.

The twin blasts at the race took place almost simultaneously and about 100 yards 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.

Cellphone service was shut down in the Boston area to prevent any possible remote detonations of explosives, a law enforcement official said. The official was speaking 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.

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

“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.”

About two 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 line. Another explosion could be heard a few seconds later.

The Boston Marathon said that bombs caused the two explosions and that organizers were working with authorities to determine what happened. The Boston Police Department said two people were killed and 23 others injured. At least eight of the wounded were in critical condition, according to hospitals.

A senior U.S. intelligence official said the two other explosive devices found nearby were being dismantled. The official spoke on condition of anonymity because he was not authorized to discuss the findings publicly.

A third explosion was heard about an hour after the first two after authorities warned spectators to expect a loud noise from a water cannon that police apparently were using to destroy one of the devices.

Competitors and race volunteers were crying as they fled the chaos. Authorities went onto the course to carry away the injured while race stragglers were rerouted away from the smoking site.

Roupen Bastajian, a 35-year-old state trooper from Smithfield, R.I., had just finished the race when they put the heat blanket wrap on him and he heard the first blast.

“I started running toward the blast. And there were people all over the floor,” he said. “We started grabbing tourniquets and started tying legs. A lot of people amputated. … At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing.”

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.

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.

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.

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