Doctors in emergency rooms across the city scrambled to treat the wounded.
Three people, including an eight-year-old boy, were killed, while at least 17 people are reported to be in a critical condition, reported CNN.
Many patients were treated for cuts and scrapes.
Doctors have also been "pulling ball bearings out of people in the emergency room".
Medical teams have carried out at least 10 amputations and treated many leg injuries.
Peter Fagenholz, a trauma surgeon at Massachusetts General Hospital, said that he treated many of the 29 patients who arrived at the hospital with shrapnel wounds.
Many of the victims were hit with "a lot of small, metal debris", he said.