An emergency room doctor at Massachusetts General Hospital told ABC News that the hospital had performed several amputations, particularly on victims whose legs were injured. Many of the victims were runners still wearing numbers on their shirts, the doctor said.
He described the injuries as "shrapnel-type wounds" possibly caused by "pipe bombs," though police have not confirmed that description.
Earlier, a trauma nurse from Massachusetts General Hospital told ABC News that medical workers had set up a temporary morgue at a medical tent at the road race and were treating patients with severed limbs and children with severe burns.
In the immediate aftermath of the explosions, Boston EMS personnel could be seen shuttling the injured out of the blast area on wheelchairs. Several of the victims were bleeding from the face.
A doctor who was in the medical tent about 150 yards away from the explosion said it looked like a "war zone," with "lots of blood," and said that all physicians were told to go to the scene and help the injured.
Boston police set off a third explosion before 4 p.m. and were sweeping the area, checking dozens of bags left behind by runners who evacuated the area after the explosions. Officials also tested for chemicals to help determine what kind of device was used, according to police.
Attorney General Eric Holder was in touch with the FBI in Boston and President Obama was notified of the blasts. All of Boston's police force was ordered to report to duty.
Security precautions were taken elsewhere beyond Boston. In Washington, D.C., Pennsylvania Avenue in front of the White House was closed to pedestrians and there was heightened security.
In Boston, police told people in area of the blasts to avoid trash cans, according to witnesses.
The explosions erupted on what is usually a festive day in Boston. It is designated Patriots Day and most offices are closed for the celebration and the marathon.
Debris from the explosions could be seen scattered throughout the spectators' stands and finish line area of the marathon as emergency personnel cleared the area.
Video of the explosions showed plumes of white smoke pouring into the air above the street where runners were.
More than 26,00 runners were registered to compete in this year's marathon. The marathon clock was at shortly after four hours at the time of the explosions, which is the average time it takes runners to complete the Boston race, potentially putting the greatest number of competitors at risk.