The explosions, 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.
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.
The Marriott hotel at Copley Place was evacuated as a precaution.
The Lenox Hotel was also evacuated as a precaution, the Boston Globe reported.
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.”
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.