Two bombs struck near the finish line of the Boston Marathon on Monday, killing two, including an 8-year-old boy.
The blasts threw people to the ground, injuring dozens.
The Boston Globe reported that more than 100 people were being treated at area hospitals, including several 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.
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.
Numerous hospitals treat the wounded
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. Boston Medical Center reported 20 patients from the marathon, most with lower leg injuries.
Boston Children's Hospital was treating seven injured people -- six children and a parent of one of the children. Their conditions range from "good to serious," a hospital spokeswoman said.