(CNN) -- Two bombs struck near the finish line of the Boston Marathon on Monday, turning a celebration into a bloody scene of destruction.
The blasts threw people to the ground, killing two and injuring dozens.
Hospitals reported at least 110 people being treated, at least eight of them in critical condition and 14 in serious condition. At least eight of the patients are children.
"Any responsible individuals, any responsible groups, will feel the full weight of justice," President Barack Obama vowed.
Boston "is a tough and resilient town," he said, adding that Americans will stand by Bostonians "every single step of the way."
'Like a huge cannon'
The terrorist attack, near the marathon's finish line, triggered widespread screaming and chaos, shattered windows and barricades and sent smoke billowing into the air at Copley Square.
They were about 50 to 100 yards apart, officials said.
"It felt like a huge cannon," a witness told CNN about one of the blasts.
Photos from the scene showed people being carried away on stretchers. One man in a wheelchair had blood all over his face and legs.
The bombs shook buildings, sending people to seek shelter under tables, witnesses said.
Federal authorities are classifying the bombings as a terrorist attack, but it's not clear whether the origin was domestic or foreign, a federal law enforcement official with knowledge of the investigation said.
A federal law enforcement official told CNN that both bombs were small, and initial tests showed no C-4 or other high-grade explosive material, suggesting that the packages used in the attack were crude explosive devices.
Another explosive device found
Authorities in Boston found at least one other explosive device that they were dismantling, Boston Police Commissioner Ed Davis said.
Davis said a third blast at the John F. Kennedy Presidential Library was believed to be related to the marathon bombings, but police later said that incident was believed to be fire-related. The library said all staff and visitors are safe.
It was unclear who may have planted the marathon bombs. There were no credible threats before the race, a state government official said.
There is no suspect in custody, but many people are being questioned, Davis said.
As authorities searched the scene, numerous suspicious packages were found, possibly because people fled the area, leaving items behind. Investigators were checking them.
The Marriott hotel at Copley Place was evacuated as a precaution.
The Lenox Hotel was also evacuated as a precaution, the Boston Globe reported.
Crowds were in the area watching the runners take part in the world's oldest annual marathon.
It was also Patriot's Day, commemorating the opening battle of the Revolutionary War.
Within seconds, the festive occasion turned into devastation.
"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."
U.S. Attorney General Eric Holder spoke with FBI Director Robert Mueller and U.S. Attorney Carmen Ortiz, a Justice Department official said.
Holder has directed the full resources of the Justice Department to be deployed to ensure the matter is fully investigated, the official said.
The Federal Aviation Administration placed a flight restriction over the site of the blasts.
Other cities, including New York and Washington, tightened security as a result. Following standard protocol, the White House cleared out an area in front of the West Wing.
"If you see something, say something," Mark Boughton, mayor of Danbury, Connecticut, wrote on Twitter. "All cities will be on a heightened state of alertness per Homeland Security protocols."
Mike Baingon, who works at the Atlantic Fish Company in Boston, said an explosion took place in front of the restaurant and that he was right by the front door at the time.
The explosions occurred at about 2:45 p.m., more than two hours after the first of the race's nearly 27,000 runners had crossed the finish line, CNN Producer Matt Frucci reported.
The race was halted as was subway service into the area.
Runners east of Massachusetts Avenue were directed to Boston Common; those west of Massachusetts Avenue were directed to Kenmore Square, the state's emergency management agency said.
Troops from the Massachusetts National Guard were assisting police as well.