At least three people were killed and 23 others wounded when two explosions struck near the finish line of the Boston Marathon on Monday, sparking scenes of panic, police said.

Police did not immediately say whether the explosions were part of a terrorist attack, but marathon organizers said it was a twin bombing and media outlets reported that other unexploded devices had been found nearby.

The blasts left a street littered with blood and debris, as spectators screamed and fled, paramedics hauled away stretchers and police rushed in, according to witnesses, one of whom saw a man with his lower limbs blown off.

Boston Marathon organizers said on Facebook that "two bombs" exploded near the finish line, without providing a source for the information.

"We are working with law enforcement to understand what exactly has happened," it said.

NBC News, citing officials, earlier reported that police had found "multiple explosive devices" in Boston, raising the possibility of a coordinated attack.

The twin explosions come more than a decade after nearly 3,000 people were killed in airplane strikes on New York, Washington and Pennsylvania on September 11, 2001.

At a hastily convened press conference, Massachusetts Governor Deval Patrick described a "very fluid situation" as he and other officials implored anyone with information about the incidents to call local tip hotlines.

Boston Police Chief Ed Davis said there was a third explosion at the John F. Kennedy Presidential Library, but the library later said there was an electrical fire that had been extinguished without causing any injuries.

A video clip posted online appeared to show one of the marathon explosions going off, apparently several feet back from the barricades and the line of national flags separating the race spectators and the street.

One runner is seen staggering and then falling, while others keep going for a few more steps. Volunteers in yellow jackets cover their ears at the sound of the blast.

"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."

"It was bad, it was fast," he said. "There was a gigantic explosion... we felt wind on our faces... Police were saying: 'Get out, get out, leave, leave there may be more bombs.'"

Nineteen wounded people were taken to Massachusetts General Hospital, including six who required emergency surgery, according to Alasdair Conn, head of emergency services.

He added that several of the injured required "traumatic amputations" at the scene or at the hospital.

 Boston Medical Center said it received 20 patients, including two children, from the site of the marathon explosions. "Most of these patients have lower leg injuries," it said in a brief statement.

The Boston Police Twitter account had earlier said two people were killed and 23 wounded. Officials at the press conference declined to discuss the exact number of casualties.

US President Barack Obama was notified about the incident, and his administration was in contact with state and local authorities, a White House official said.

Obama called Patrick and Boston mayor Tom Menino to express his concern for the injured and to offer support. He was to make a statement at 2210 GMT.

Security was stepped up in New York and Washington, as well as in Los Angeles and San Francisco. 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.

"Praying for those at the Boston Marathon today," said one of the US senators from Massachusetts, Elizabeth Warren. The Senate observed a moment of silence, and the House of Representatives was to follow suit.

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. The video clip of the blast showed the marathon timeclock at 4:09:44