At least 17 people were critically injured, police said. At least eight children were being treated at hospitals. In addition to losing limbs, victims suffered broken bones, shrapnel wounds and ruptured eardrums.
At Massachusetts General Hospital, Alasdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."
Eight-year-old Martin Richard was among the dead, according to a person who talked to a friend of the family and spoke on condition of anonymity. The person said the boy's mother and sister were also injured as they waited for his father to finish the 26.2-mile race.
A candle burned on the stoop of the family's single-family home in the city's Dorchester section Tuesday, and the word "Peace" was written in chalk on the front walk.
Neighbor Betty Delorey said Martin loved to climb the neighborhood trees, and hop the fence outside his home.
Tim Davey of Richmond, Va., was with his wife, Lisa, and children near a medical tent that had been set up to care for fatigued runners when the injured began arriving. "They just started bringing people in with no limbs," he said.
"Most everybody was conscious," Lisa Davey said. "They were very dazed."
The Boston Marathon is one of the world's oldest and most prestigious races and about 23,000 runners participated. Most of them had crossed the finish line by the time the bombs exploded, but thousands more were still completing the course.
The attack may have been timed for maximum bloodshed: The four-hour mark is typically a crowded time near the finish line because of the slow-but-steady recreational runners completing the race and because of all the friends and relatives clustered around to cheer them on.