When the smoke cleared after the blasts in Boston's Back Bay section, dozens of victims lay in the street, some unconscious, some grievously injured, including some whose limbs had been torn off by the blast, Boston Globe reported.
The dead included an 8-year-old boy, the newspaper said citing two law enforcement sources. Boston Children's Hospital reported that those treated there included a 9-year-old girl, a 7-year-old boy, a 12-year-old, and a 2-year-old.
Chaos erupted as near-simultaneous blasts struck about 90 metres apart, the first on Boylston Street near the photo bridge that marks the finish line, CTV News said. The explosion knocked some spectators and runners off their feet.
Paramedics and National Guard members who were working the race rushed to tear down fencing that cordoned off the spectator area from the course in order to reach and treat the injured.
The blasts threw people to the ground. Boston Police Commissioner Ed Davis said Monday night that the death toll had risen to three. Scores were injured at the scene.
Hospitals reported at least 141 people are being treated, with at least 17 of them in critical condition and 25 in serious condition, CNN said. At least eight of the patients are children.
Investigators warned police to be on the lookout for a "darker-skinned or black male" with a possible foreign accent in connection with the attack, the channel said citing a law enforcement advisory obtained by it.
The man was seen with a black backpack and sweatshirt and was trying to get into a restricted area about five minutes before the first explosion, the lookout notice states.
Also, a Saudi national with a leg wound was under guard at a Boston hospital in connection with the bombings, but investigators cannot say he is involved at this time and he is not in custody, a law enforcement official said Monday evening, it said.
Authorities in Boston found at least one other explosive device that they were dismantling, Davis said.
In a television address President Barack Obama pledged the full resources of the federal government in helping Boston and in investigating the bombings and said those responsible would feel the "full weight of justice."
"The American people will say a prayer for Boston tonight," he 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."
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's 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.