Two people were confirmed dead and local media reported that more than 100 people were treated in hospital. The Boston Globe reported that some had to have limbs amputated.
There were no initial suspects or groups claiming the incident, though there were reports that additional devices had been found and defused. Fox News reported that a “person of interest” had been taken into custody, but there were no details beyond that.
One runner, a Rhode Island state police officer, said he saw at least two dozen people with very serious injuries, including missing limbs.
Competitors and race volunteers were crying as they fled the chaos. Bloody spectators were being carried to the medical tent that had been set up to care for fatigued runners.
Pictures emerged on Twitter showing casualties lying on the pavement on Boylston St and debris blowing around them.
Another photo appeared to show participants running down the street at the moment an explosion created a fireball, sending smoke into the air.
About two hours after the winners crossed the finish line, there was a loud explosion on the north side of Boylston St, just before the photo bridge that marks the line. Another explosion was heard seconds later.
Marathon organisers said that bombs caused the two explosions and that organisers were working with authorities to determine what happened.
The Boston Police Department confirmed two people were killed.
Authorities went onto the course to carry away the injured while stragglers in the 26.2-mile race were rerouted away from the smoking site.
Roupen Bastajian, a 35-year-old state trooper from Rhode Island, had just finished the race when they put the heat blanket wrap on him and he heard the first blast.
“I started running toward the blast. And there were people all over the floor. 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.”
A Boston police officer was wheeled from the course with a leg injury that was bleeding.
“There are a lot of people down,” said Frank Deruyter, a competitor from North Carolina.
He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, as blood gushed from her leg.
Smoke rose from the blasts, fluttering through the national flags lining the route. TV helicopter footage showed blood staining the pavement in the popular tourist area known as the Back Bay.
“There are people who are really, really bloody,” said Laura McLean, a runner from Toronto, who was being treated in the medical tent for dehyd-ration when she was pulled out to make room for blast victims.
Cherie Falgoust was waiting for her husband, who was running the race.
“I was expecting my husband any minute. I don’t know what this building is… it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don’t know what it was. I just ducked.”
Runners who had not finished were diverted down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.
The nearby Prudential Tower, the city’s second-tallest building with an upmarket shopping mall on the ground, was evacuated, with the luxury Mandarin Oriental hotel, according to media reports.
Race day got started with 26 seconds of silence in honour of the victims of the December school shooting in Connecticut. Just over two hours later, the lead runners passed the Mile 26 marker, which was decorated with the Newtown, Connecticut, seal and dedicated to those killed there.
The annual marathon takes place on Patriot’s Day, which celebrates the evacuation of Boston by the British in the American Revolution.