“There were two bombs that exploded near the finish line in today’s Boston Marathon. We are working with law enforcement to understand what exactly has happened,” the Boston Athletic Association, the organization that runs the marathon, a magnet for runners from all over the world, said in a statement. A Globe reporter saw people fall injured at two spots on Boylston Street.

At least three people were killed and at least 140 others were injured this afternoon as two powerful explosions detonated in quick succession near the Boston Marathon finish line in Boston’s Back Bay section, transforming a scene of athletic celebration into bloody chaos.

When the smoke cleared, dozens of victims lay in the street, some unconscious, some grievously injured, including some whose limbs had been torn off by the blast. The dead included an 8-year-old boy, according to two law enforcement sources briefed on the investigation.

President Barack Obama vowed that whoever planted the explosives would be found and brought to justice.

“We still do not know who did this or why. And people shouldn’t jump to conclusions before we have all the facts,” he told reporters at the White House shortly after 6 p.m. “But make no mistake, we will get to the bottom of this … Any responsible individuals, any responsible groups will feel the full weight of justice.”

The blast came on Patriots Day, Obama noted, which is a state holiday in Massachusetts that celebrates the beginning of the American Revolution.

Blood and broken glass covered sidewalks in the area after the blasts occurred at about 2:50 p.m.

“It was just immediately [evident] there were injuries, right in the middle of the spectator crowds,” said boston.com sports producer Steve Silva, who was on the scene, covering the race. “There was blood everywhere; there were victims being carried out on stretchers. I saw someone lose their leg. People are crying. People are confused.”

At least 120 people were brought in for treatment at seven area hospitals. Brigham and Women’s Hospital received the most, with 26 treated, including two in critical condition, hospital officials said

Asked if the explosions were a terrorist act, Boston Police Commissioner Edward F. Davis said, “We’re not being definitive on this right now, but you can reach your own conclusions based on what happened.”

No suspects have been arrested, Davis said. “We’re questioning many people, but there is no one in custody at this point,” he said.

The Federal Aviation Administration said it had implemented a no-fly zone around an area of Boston at the request of law enforcement officials and temporarily stopped planes on the ground at Logan International Airport to change the runway configuration.

Davis said the attacks had occurred without warning. “There was no specific intelligence” suggesting an attack would take place, he said.

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