Blood and broken glass covered sidewalks in Boston's Back Bay section where the suspected terrorist blasts occurred at about 2.50 p.m. Monday, Boston Globe reported.
Immediately after the explosions, some of the wounded could be seen to have lost limbs; others lay unconscious. The dead in the explosions included an eight-year-old boy, the local daily said citing a law enforcement source.
In a TV address, Obama pledged the full resources of the federal government in helping Boston and in investigating the bombings.
"The American people will say a prayer for Boston tonight," Obama said.
The president cautioned that authorities were still investigating and that people should not jump to any conclusions before all of the facts are learned.
"But make no mistake," Obama said. "We will get to the bottom of this and we will find out who did this. We'll find out why they did this and we will hold them accountable."
Individuals and groups responsible would feel the "full weight of justice", he said.
The blast came on Patriots Day, Obama noted, which is a state holiday in Massachusetts that celebrates the beginning of the American Revolution.
Obama stopped short of calling the incident an act of terrorism.
Federal authorities are classifying the bombings as a terrorist attack, CNN reported citing a federal law enforcement official.
But it was not clear whether the origin was domestic or foreign, the official 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.
At least 115 people were brought in for treatment at seven area hospitals. Brigham and Women's Hospital received the most, with 26 treated, including two in critical condition, the Globe said.
Boston Police Commissioner Edward F. Davis said no suspects have been arrested so far.
"We're questioning many people, but there is no one in custody at this point," he was quoted as saying by the Globe.
The Federal Aviation Administration said it had implemented a no-fly zone around an area of Boston at the request of law enforcement officials and temporarily stopped planes on the ground at Logan International Airport to change the runway configuration.
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.
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.