At least two people were killed and more than 60 wounded when two explosions struck near the finish line of the Boston Marathon, sparking scenes of panic, police said.

US media are reporting eight people – including two children – are “absolutely critical” condition at Massachusetts General Hospital.

The Boston Globe is reporting that two law enforcement sources briefed on the Boston Marathon bombing investigations have confirmed one of the two people who died in the attack was an eight-year-old boy.

At least 125 people were treated at seven hospitals in the Boston area. While no suspects have been arrested, “many people” are being questioned,

”This is the sort of carnage you expect to see in war,” Alasdair Conn, chief of emergency services at Massachusetts General Hospital, said at a news briefing.

Boston police confirmed the toll of dead and wounded on its Twitter feed, however the New York Post reported as many as 12 people had died with up to 50 injured and the Boston Globe said 90 people were injured.

Police did not immediately say if the bombings were part of a terrorist attack.

Australian athletes Benita Willis, Jeffrey Hunt and Kurt Fearnley escaped the carnage by finishing before the blasts. Hunt finished eighth and Fearnley fifth in the wheelchair category.

Local radio said the first explosion took place near a sports store and the second went off near a viewing stand. It was not immediately clear whether the explosions were accidental or the result of a terrorist attack.

“There were two bombs that exploded near the finish line in today’s Boston Marathon,” race organisers said on the event’s Facebook page, without providing a source for the information.

“We are working with law enforcement to understand what exactly has happened.”

Boston police confirmed there had been a third explosion in the city while a fourth device had reportedly been found but safely detonated.

Police Commissioner Edward Davis says authorities aren’t certain that the explosion at the JFK Library was related to the other blasts, but they’re treating them as if they are.

David says there are no injuries stemming from the third explosion. He urged people to stay indoors and not congregate in large groups.

US President Barack Obama said it was not clear who was responsible but those behind the attack would be found.

US president Barack Obama said at a news conference that while authorities still did not know who was responsible for the bombing, he promised ”we will get to the bottom of this”.

”We will find out who did this and hold them accountable,” he said.

”Boston is a tough and resilient town … and will move forward as one proud city.”

Reports of Marathon Runners that crossed finish line and continued to run to Mass General Hospital to give blood to victims #PrayforBoston

Security was stepped up in New York and Washington, sites of the 9/11 attacks. In the Big Apple, police said they were boosting security at hotels and “other prominent locations in the city.”

The blasts in Boston rattled US markets, sending the Dow and the S&P 500 down at the close.

The president called Boston mayor Tom Menino and Massachusetts Governor Deval Patrick to express his concern for the injured and to offer support.

Vice President Joe Biden reacted to the incident in real time, as he was on a conference call with gun reform advocates at the White House.

“As I am speaking here, they have just turned on the television in my office, and apparently there has been a bombing — I don’t know any of the details of what caused it, who did it.”

“Our prayers are with those people in Boston who are suffering injury.”

Shortly after the Boston blasts, Secret Service officers cordoned off an area adjacent to the fence at the front of the North Lawn of the White House on Pennsylvania Avenue, which would normally be thronged with crowds of tourists.

Zara Bielkus, a 30-year-old spectator from Boston, said she heard two explosions seconds apart. Police then locked down the area.

“When we heard them, everyone looked at each other and went very quiet, and within a minute police came,” she said.

The owner of a hotel near the finish line said there were bloody limbs on his patio in the aftermath of the explosion.

“We saw people with their legs blown off,” Mark Hagopian, owner of the Charlesmark Hotel, told AFP from the basement of a restaurant where he had sought shelter.

“A person next to me had his legs blown off at the knee — he was still alive.”

The Boston Marathon is one of the biggest annual athletic events held in the United States, with nearly 27,000 racers who must qualify to compete and tens of thousands of spectators.

The race attracts world-class athletes, most of whom would have likely completed the race a couple hours before the blast went off. Footage of the blast showed the marathon timeclock at 4:09.