A source briefed on the investigation said authorities found three other devices that did not explode, including one in the Boston suburb of Newton. Bomb disposal teams were examining discarded bags and parcels left at the scene of the blasts.
There was no word on the motive or who may have launched the attack, and authorities said there was no immediate claim of responsibility.
U.S. President Barack Obama said in televised remarks that the United States does not know "who did this or why," but he vowed that whoever is responsible "will feel the full weight of justice."
While Obama did not call the explosions an attack, a White House official said any event with multiple explosive devices, as this appears to be, "is clearly an act of terror, and will be approached as an act of terror."
Television footage showed scenes of confusion, streets littered with debris and blood, paramedics carrying stretchers, and damage to nearby buildings.
Bloody spectators, including some with severed limbs, were carried to the medical tent that had been set up to care for fatigued runners from the race.
The blasts occurred about two hours after the first of the race's more than 23,000 runners had crossed the finish line. About 17,600 participants finished before the explosions. The competition was halted after the bombs went off, as was subway service to the area.
The Federal Aviation Administration placed temporary flight restrictions in the airspace over the site of the initial twin explosions near the finish line of the marathon. The restrictions do not affect commercial air operations at Boston's Logan Airport.
Cities worldwide stepped up security following the explosions.
In Britain, police said they are reviewing security plans for Sunday's London Marathon, the next major international marathon. London is considered a top target for international terrorists.
New York City officials said police have increased security around landmarks in Manhattan, including near prominent hotels, in response to the blast. Washington, D.C. and Los Angeles also are on a heightened state of alert.
Officials in Boston said an electrical fire that broke out at the John F. Kennedy Library a few kilometers from the marathon's finish line was not related to the bombings.
Shortly after the explosions, the U.S. Secret Service shut down Pennsylvania Avenue outside the White House, cordoning off the area with yellow police tape. But the White House is not on lockdown.
President Obama called Boston Mayor Tom Menino and Massachusetts Governor Deval Patrick and told them his administration will provide whatever assistance is necessary in the investigation and response.
Boston is a major metropolis located in the northeastern U.S. The marathon is a significant event the city hosts every year. It attracts runners and spectators from all over the world.
UPDATE: CNN - The death toll from two explosions near the Boston Marathon finish line has risen to three, Police Commissioner Ed Davis said.
