Two dead as explosions hit Boston Marathon finish line

President Barack Obama said the full resources of the federal government are being mobilised to investigate the deadly explosions today in Boston and he vowed those responsible will brought to justice.

Two bombs exploded near the finish of the Boston Marathon, killing two people, injuring 23 others and sending authorities rushing to aid wounded spectators at America’s oldest and most prestigous marathon.

One runner, a Rhode Island state police officer, said he saw at least two dozen people with very serious injuries, including missing limbs.

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A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square. Photograph: Jim Rogash/Getty Images

About two hours after the winners crossed the finish line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the line. Another explosion could be heard a few seconds later.

Boston Marathon organisers said they were working with authorities to determine what happened. The Boston Police Department confirmed two deaths and at least 23 injuries. A number of the injured are understood to be "gravely ill".

Mr Obama said those responsible will feel the "full weight of justice". The US President told a news conference at the White House: "The American people will say a prayer for Boston tonight, and Michelle and I send our deepest thoughts and prayers to the families of the victims in the wake of this senseless loss.

"Any responsible individuals, any responsible groups will feel the full weight of justice." He said security will be increased across the country "as necessary." While Mr Obama didn't use the word terrorism to describe the attack, an administration official said the bombings are being investigated as an act of terror. The president was notified of the incident today about 3pm, 10 minutes after the twin blasts detonated at the end of the race.

Competitors and race volunteers were crying as they fled the chaos. Bloodied spectators were being carried to the medical tent that had been set up to care for fatigued runners. Authorities went onto the course to carry away the injured while stragglers in the 26.2-mile (42-kilometre) race were rerouted away from the smoking site.

Dublin runner Gerry Carr who crossed the finishing line just before the first explosion described the scene. "I was about 100 yards past the finishing line when the first bomb exploded," he said. "I was just talking to an American I'd been running with and we were congratulating each other and we heard this big bang followed by another one about 30 seconds later."

The official website of the Boston Marathon lists 50 entrants resident in Ireland and 108 entrants with Irish citizenship. As yet, there are no indications of any injuries in their midst.

Roupen Bastajian, a 35-year-old state trooper from Rhode Island, 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 fire at another location in Boston which police initially said they were linking to the explosions is no longer being connected, police said.

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. TV helicopter footage showed blood staining the pavement in the popular shopping and tourist area known as the Back Bay.

“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.”

Cherie Falgoust was waiting for her husband, who was running the race.

Those 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.

Earlier, the administration said it is in contact with state and local authorities and the president directed his administration to provide whatever assistance is necessary in the investigation and response.

Vice president Joe Biden was on a conference call with gun control activists when staffers turned on televisions in his office to view coverage of the explosions. Mr Biden said during the call that his prayers were with those who suffered injuries.

Boston’s Prudential Tower, the city’s second-tallest building with an upmarket shopping mall on the ground, was evacuated, with the luxury Mandarin Oriental hotel, according to media reports.

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.

Shortly after the explosions, secret service officers closed down Pennsylvania Avenue outside the White House, cordoning off the area with yellow police tape. Several secret service patrol cars also blocked off the entry points to the road.

The White House was not on lockdown and tourists and other onlookers were still able to be in the park across the street from the executive mansion.

The Federal Aviation Administration warned pilots that it had created a no-fly zone over the site of the explosions in Boston.

The agency said in a notice issued Monday about an hour after the explosions that a no-fly zone with a 3.5-mile (5.6-km) radius has been created over 811 Boylston Street. The zone is limited to flights under 3,000 feet (914 metres), which is lower than most airliners would fly except when taking off or landing.

The notice said the no-fly zone is effective immediately and will remain in effect until further notice. Pilots planning flights were urged to call their local flight service station.

Homeland security secretary Janet Napolitano directed her agency to provide "whatever assistance" necessary.

AP