A pair of explosions in quick succession near the finish line of the annual Boston Marathon on Monday killed three people, according to news reports. At least 140 others were injured, some critically.

As smoke cleared in the packed city street where a grandstand full of well-wishers was to have greeted marathon runners, dozens of victims could be seen, the Boston Globe reported. Some of those lying unconscious near Copley Square had been grievously hurt, including some whose limbs were blown off by the explosions. Sidewalks were covered in blood and broken glass.

One of the dead was an 8-year-old boy, the newspaper said, citing law enforcement sources. Eight hospitals said they were treating dozens of victims, including some children. At least 15 people were said to be in critical condition, the Associated Press reported.

US President Barack Obama vowed that whoever was responsible for the blasts 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," Obama told reporters at the White House about three hours after the 2:50 pm incident in Boston. "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."

A man who described himself as a service technician hired to set up and dismantle television equipment for the marathon described the scene for a China Daily reporter at the scene.

"It was something you see out of a science-fiction movie," said the man, who gave only his first name, Joe. "We set up all the circuits, you know, for all the TV stations. They were coming around on live. We were just waiting for them to break down. Once the elite runners finish, they kind of wind down their coverage."

Joe, who was working indoors, said the blasts prompted him to look outside for co-workers and make sure they were OK. They were, so Joe and other race-day workers gathered in a nearby library. When they left, police had begun clearing the streets and caring for the injured in the chaotic aftermath.

"That's when I saw they were taking people away. It was just - it was horrible," said Joe, who had seen a man whose legs apparently had been severed by the blast.

The Boston Marathon, in its 117th year, fell as always on Patriots Day, a state holiday in Massachusetts that commemorates the beginning of the American Revolution with the battles of Lexington and Concord in April 1775. The annual race attracts thousands of competitors from around the world, including this year a team of 15 executives and employees of China Vanke Co, the country's biggest property developer.

"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, which organizes the marathon, said in a statement.

The Boston office of the US Federal Bureau of Investigation said the situation "remains fluid and it remains too early to establish the cause and motivation" of Monday's explosions.

The Globe quoted Boston's police commissioner, Edward Davis as saying when asked if the blasts were a terrorist act: "We're not being definitive on this right now, but you can reach your own conclusions based on what happened."

Davis said no suspects had been arrested. "We're questioning many people, but there is no one in custody at this point," he said.

Intelligence officials were quoted as telling Dow Jones Newswires that the Boston explosions were being treated as a domestic incident. "Raw intelligence of a domestic nature" was being gathered and sent to the FBI, a senior intelligence official said.

The official said US intelligence agencies would continue working in support of the FBI unless the investigation turned up some connection to a foreign entity.

The intelligence picture was changing by the minute, the official said, according to Dow Jones.

Authorities had no indication of a potential threat to the marathon beforehand, the official said.

The US 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, Boston's police chief, said the attacks had occurred without warning, according to the Globe. "There was no specific intelligence" suggesting an attack would take place, he said.

Davis had initially said there had been a third explosion at the nearby John F Kennedy, but the commission later said the incident might have been "an incendiary device or a fire" and that it wasn't linked to the marathon blasts.

Obama, speaking in Washington, said he had directed the full resources of the US government to "investigate and respond" to Monday's explosions and that he had assured Massachusetts Governor Deval Patrick and Boston Mayor Thomas Menino that "every single federal resource necessary" would be available to them.

"The American people will say a prayer for Boston tonight," the president said. "We will find out who did this and we will hold them accountable."