Police said that there were two simultaneous explosions at 2:50pm local time, about 50 to 100 yards apart and that each scene resulted in multiple casualties.
Runners were still finishing the race as the blasts hit and initial reports suggest that many of those injured were spectators at what is one of the most popular sporting events in the US.
One of the dead was an eight-year-old boy, according to US media reports. At least 17 people are critically wounded and the injuries include several amputations.
Bloodied victims were initially rushed to a medical tent set up to care for fatigued runners. Police later confirmed that “all the victims have been removed”.
A televised statement from US president Barack Obama said that authorities “continue to monitor and respond to the situation as it unfolds”.
Obama continued: “We don’t yet have all the answers. Multiple people have been wounded, some gravely, in explosions at the Boston Marathon.
“We still do not know who did this and why. And people should not jump to conclusions. But make no mistake. We will found out who did this and why they did this. Any responsible group or individuals will feel the full weight of justice.
“We’re still in the investigation stage. But I just want to reiterate: We will find out who did this and we will hold them accountable.”
New York police stepped up security around landmarks and prominent hotels in response to the blasts.
Following the blasts, British police have said that security for the London Marathon on Sunday will be reviewed.
London Marathon chief executive Nick Bitel said: “We are deeply saddened and shocked by the news from Boston.
“Our immediate thoughts are with the people there and their families. It is a very sad day for athletics and for our friends and colleagues in marathon running.
“Our security plan is developed jointly with the Metropolitan Police and we were in contact with them as soon as we heard the news.”
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news