BOSTON — More than 130 people were injured today as multiple explosions rocked the Boston Marathon in a “coordinated” terror attack.

Two explosions were detonated at the race’s finish line within seconds of each other, beginning at 2:50 p.m. Three other undetonated devices were later discovered by authorities.

A White House official told Bloomberg news, “Any event with multiple explosive devices — as this appears to be — is clearly an act of terror.”

Homeland Security committee member Rep. Bill Keating (D-MA) called today’s explosions a “sophisticated, coordinated event.”

WCVB in Boston is reporting that an 8-year-old boy was among those killed in Monday’s attack.

Law-enforcement sources said at least the first explosion occurred in the lobby of a nearby hotel.

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“There were two bombs that exploded near the finish line in today’s Boston Marathon,” The Boston Marathon’s official Facebook page read. “We are working with law enforcement to understand what exactly has happened.”

The airspace above the city was ordered cleared as well and was briefly designated a no-fly zone, which has since been lifted, by the FAA following the explosions.

Police confirmed a third explosion at JFK Library in Boston but officials later said it was unrelated to the marathon attack. There were no injuries reported from the third bombing.

“Fire in building is out, appears to have started in the mechanical room of new building,” The JFK Library’s Twitter account read. “All staff and visitors are accounted for and safe.”

A law enforcement official told the Associated Press that officials shut down cellular service throughout the Boston area to prevent remote detonation of explosives. Cellular service has since been restored but it is still difficult to communicate with anyone in the Boston area, according to witness reports.

Cops also issued an all-points-bulletin for a yellow Penske truck or van that tried to get access to a marathon site, claiming it had medical supplies — and then sped off or was turned away.

President Obama, was briefed about the incident shortly after the explosions occured.

The Secret Service said they have expanded the security perimeter around the White House out of “an abundance of caution.”

Obama called Boston mayor Tom Menino and Massachusetts Governor Deval Patrick to express his concern for those that were injured and make clear that his administration is ready to provide needed support as they respond to this incident.

“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,” Obama said Monday evening.

Gov. Patrick released a statement this afternoon.

“This is a horrific day in Boston. My thoughts and prayers are with those who have been injured. I have been in touch with the President, Mayor Menino and our public safety leaders. Our focus is on making sure that the area around Copley Square is safe and secured. I am asking everyone to stay away from Copley Square and let the first responders do their jobs.”

The after-effects from the explosions reverberated across the nation, including in New York City.

“This changes everything [in New York],’’ one source told The Post. “Everyone’s on high alert. Our marathon was canceled in November. Who knows if that changed someone’s plans?’’

The NYPD has ramped up its anti-terror efforts at landmarks and other potential terrorist targets “to make sure there’s no nonsense going on here,’’ a law-enforcement source said.

Officers are scouring live feed from surveillance videos throughout the city to monitor possible events, the source said.

NYPD Deputy Commissioner Paul Browne said that the NYPD has deployed its Critical Response Vehicles as well until more about the explosion is learned.

The FDNY says their units have also been advised to use elevated caution due to the explosions.

Mayor Bloomberg released a statement following the explosions.

“As law enforcement authorities investigate today’s explosions in Boston, I ask all New Yorkers to keep the victims and their families in your thoughts and prayers,” Bloomberg said. “I have spoken with Police Commissioner Ray Kelly, and the NYPD has stepped up security at strategic locations and critical infrastructure, including our subways.”

Governor Cuomo echoed Bloomberg’s sentiments and saying that state agencies have all been placed on a heightened state of alert.

Relatives of the victims of December’s school massacre in Newtown, Conn. were among those attending the race finish at a VIP area. It was not known of any of the relatives were among the casualties.

Competitors and race organizers were crying as they fled the chaos. Bloody spectators were being carried to the medical tent that had been set up to care for fatigued runners.

“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. A Boston police officer was wheeled from the course with a leg injury that was bleeding.

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

Runner Laura McLean of Toronto said she heard two explosions outside the medical tent.

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

”I was expecting my husband any minute,” she said. “I don’t know what this building is … it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don’t know what it was. I just ducked.”

About half of the runners — virutally all the elite competitors — had completed the course when the twin explosions went off.

It created a mad rush of bystanders, who frantically dashed away from the scene. Moments later, first responders ran to the wounded, as their blood stained the Boston pavement.

Smoke filled the previously crystal-clear sky and the sounds of sirens rang for blocks.

With Josh Margolin, Kirstan Conley, Andy Soltis and Post Wire Services.