Two explosions were heard near the finish line of the Boston Marathon on Monday.

According to Fox News, at least two people were killed and dozens injured, including ten amputations, following a large explosion, then smaller one, a short while later.

It happened about three hours after the race winners crossed the finish line on Boylston Street, according to WBZ-TV’s Lisa Hughes, who was near the scene covering the marathon.

There were two “incredibly powerful explosions just seconds apart,” according to Hughes.

At two spots on Boylston Street, scores of people fell and were injured. Some appeared to have lost limbs, and others were knocked unconscious.

“Somebody’s leg flew by my head,” a spectator, who gave his name as John Ross, told the Boston Herald. “I gave my belt to stop the blood.”

Video from the finish line showed screams and an enormous cloud of white smoke, and about 20 seconds between the blasts. The Associated Press reported that bloody spectators were being carried to a medical tent that had been set up to care for tired runners.

Jackie Bruno, a reporter for New England Cable News, said on Twitter that she saw people’s legs blown off.

“Runners were coming in and saw unspeakable horror,” she said.

A Fox reporter on the scene says the explosions came from the middle of a building. Police are checking to see whether explosive devices were placed in trash cans.

Hotels and restaurants are locked down while authorities secure the area. Police are still finding “secondary devices,” and a pleading with spectators to go home. The simultaneous explosions, and reports of at least one other unexploded device found near the scene – raised suspicions that the explosions, just before 3 p.m., could be terrorist attacks.

A White House official told NBC News President Obama has been notified of the explosions, the causes of which were not immediately clear, and had directed his administration to provide necessary assistance in the investigation and response.

New York City has stepped up security in the wake of the explosions.

“We’ve stepping up security at hotels and other prominent locations in the city through deployment of the NYPD’s critical response vehicles until is more about the explosion is learned,” Deputy Commissioner of the New York Police Department Paul J Browne told CNBC.

Security was also increased in Washington, D.C., at Pennsylvania Avenue in front of the White House, officials told NBC News.

This is Patriot’s Day, which commemorates the battles of Lexington and Concord, fought near Boston in 1775. Patriot’s Day is annually held on the third Monday of April.

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