BOSTON - At least four people were killed and over 100 wounded when three explosions went off in this US on Monday evening, sparking scenes of panic.

Two explosions struck near the finish line of Boston Marathon followed by a third one near the John F Kennedy Presidential Library while police sources said two other bombs were found in the marathon area before they went off.

Witnesses said the streets were littered with debris and blood, and paramedics raced off with stretchers as police locked down the area. It was apparently a terrorist attack and possibly the biggest one on the US soil since twin tower attacks in New York on September 11, 2001.

Local radio said the first explosion took place near a sports store and the second went off near a viewing stand. Local media reported the third explosion nearby, but cited police as saying it was a controlled detonation as they had found it in advance. NBC News later reported that ‘multiple explosive devices’ were found in Boston.

US President Obama was notified about the incident and he ordered full investigations into the bombings and his administration was in contact with state authorities. Former Pakistan ambassador to US Hussain Haqqani, who was among tens of thousands of spectators, had reportedly reached home safe and sound before the bombings.

As the horror of the explosions sank in, stock exchanges across the US plunged down and aviation authorities banned planes from flying over of the site of the explosion. The Marriott hotel at Copley Place and Lenox Hotel were evacuated as a precaution. Security was put on high alert in all major cities of the US including New York and Washington.

Massachusetts General Hospital said it was treating 22 injured people, six of them in critical condition. Tufts Medical Center reported that it was treating nine people. Brigham and Women’s Hospital said it received between 18 and 20 injured. Combined, that brings the number of injured to as many as 51. There are many other hospitals in the Boston area. The full number of casualties was not immediately known, Davis said.

Crowds were in the area watching the runners. “This is a horrific day in Boston,” Massachusetts Gov. Deval Patrick said in a statement. “My thoughts and prayers are with those who have been injured. I have been in touch with the president, Mayor (Thomas) 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.”

Attorney General Eric Holder instructed the Justice Department to deploy its “full resources” to the investigation into the blasts. He spoke to the head of the FBI, Robert Mueller, and to the US Attorney for the District of Massachusetts, Carmen Ortiz.

Her office is coordinating responses from the Justice Department, the FBI, the Bureau of Alcohol, Tobacco and Firearms and other law enforcement officials. Homeland Security Secretary Janet Napolitano was briefed, and she ordered her department to provide “whatever assistance is necessary” to state and local authorities, an administration official said.