Boston was on lockdown early today after two bombs exploded close to the finishing line of the city's marathon, killing at least two people and injuring as many as two dozen more.
All tourists and residents were told to remain in their homes after a third explosion at the city's JFK library.
Boston's police commissioner Ed Davis confirmed that officers were treating the attacks as an "ongoing situation".
Mobile phone networks were reportedly shut down to prevent any devices being remotely detonated, and flights were grounded at the city's Logan airport as police desperately scrambled to make sense of the confusion.
In the immediate aftermath of the marathon bombs, a plume of smoke could be seen rising above the Charles river, while video from helicopters at the scene revealed pools of blood spilled over a large area of pavement on Boylston Street.
Witnesses reported seeing people with limbs missing after the blasts.
The attacks sent waves of shock across America, which has not suffered any successful attacks since September 11, 2001.
Even before it was confirmed that the explosions were indeed deliberate, special precautions were triggered, including the closing down of Pennsylvania Avenue close to the White House in Washington to foot traffic.
Counter-terrorism police were also deployed to major landmarks in Manhattan and strategic locations across the country.
Mr Davis warned people in Boston to remain calm and stay indoors as the authorities continue to investigate a series of explosions in the city.
He said that officials did not know of any specific threats before three devices exploded.
He said that local police were getting aid from every agency in the state and the federal government after the explosions.
"Every asset of the Commonwealth of Massachusetts and the federal government is either here or is coming here," Mr Davis said.
Speaking at a news conference at a hotel near the finish line of the Boston Marathon, Mr Davis said that anything left on the streets was being treated as a suspicious device.
A number of eyewitnesses described the scenes of carnage.
Bruce Mendelsohn, war veteran, said: "I heard the explosion, I saw the smoke, and felt a secondary, smaller explosion five seconds later.
"I exited the building [directly above the blast]; the street was smeared with blood and shrapnel. I saw wounds that were commensurate with a ground-level explosion.
"It was terrifying and people are rightly calling it Boston's 9/11. The scene went from festive to chaotic and bloody immediately. The explosion came from a building and it blew out the glass, there was broken glass strewn all over the front.
"Once the police had the scene under control, they cordoned everything off and whisked everyone away," he added.
Josh Cox, elite runner and US commentator, said: "There were bodies, people laying on the ground. Some runners who had just finished, who had just gotten by it, were crying, understandably. It's tragic, absolutely tragic.
"It was a massive boom. When the first explosion went, the whole ground shook. I didn't think bomb, you think there was a transformer or generator -- I took a picture. Right as I took the picture, the next one went off. Once the second one came, everyone was freaking out."
Fatma Tanis, a journalist, said: "I was at Copley Square, 300ft from the explosion. I was interviewing one of the runners and my camera was rolling at the same time.
"Suddenly, I heard a loud noise, like a cannon going off. The ground shook. It was like a heavy falling noise.
"Then I saw a fireball and smoke. There was mass confusion. People were scared. Closer to the scene, people had cuts on faces; others were running. I walked towards the area of the explosion. I saw blood on the floor and lots of bodies. The second explosion was not as loud."
Stuart Singer, British marathon runner, said: "I finished the marathon . . . I met up with my wife just at the end and we started walking back to our hotel and then we heard the explosions and lots of sirens.
"We didn't think too much of it, we just thought 'what was that bang?' and carried on. Then we saw the news. It was just horrific, to be honest.
"There's loads and loads of helpers handing out sheets for runners, handing out drinks -- there's hundreds and hundreds of people there -- it must be complete chaos. For the last mile coming in, there are literally thousands of people."