Competitors and race volunteers were crying as they fled the chaos. Authorities went onto the course to carry away the injured, while race stragglers were rerouted away from the smoking site.
Roupen Bastajian, a state trooper from Smithfield, R.I., had just finished the race when he heard the blasts.
"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated. ... At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."
Bruce Mendelsohn was attending a post-race party in an office building just above the Boston Marathon finish line when an explosion knocked him to the floor. The former Army medic rushed outside to find blood, glass and debris everywhere. He began applying pressure to gruesome wounds.
"This stuff is more like Baghdad and Bombay than Boston," said Mendelsohn, who works at the Massachusetts Institute of Technology. "It was pretty terrifying."
The bombings forced an abrupt end to the race. Cellular service in the area went down, making it difficult for many to find loved ones on the course.
The Boston Athletic Association which organizes the event posted this statement on its website: "Today is a sad day for the City of Boston, for the running community, and for all those who were here to enjoy the 117th running of the Boston Marathon. What was intended to be a day of joy and celebration quickly became a day in which running a marathon was of little importance."
The race honored the victims of the Newtown, Conn., shooting with a special mile marker in Monday's race. Boston Athletic Association president Joanne Flaminio previously said there was "special significance" to the fact that the race is 26.2 miles long and 26 people died at Sandy Hook Elementary school.