The explosions occurred around 3pm local time (5am EST) with CCTV image of the scene showing two blasts coming just metres apart from each other as competitors crossed the finishing line.
Two other undetonated devices were found and subsequently disposed of safely by local authorities.
It is being reported that the explosion knocked runners and spectators alike to the ground, injuring many seriously.
Photographs coming out of Boston have also shown the carnage, with the street around Copely Square apparently awash with blood.
Of the 105 so far confirmed injured, 77 have been hospitalised with injuries, some of them missing limbs.
‘‘This is the sort of carnage you expect to see in war,’’ said Alasdair Conn, chief of emergency services at Massachusetts General Hospital, where victims were taken.
A 35-year old state trooper from Rhodes Island has also reported the severity of some of the injured to The Sydney Morning Herald.
‘‘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.’’
The New York Post is reporting that a “bomber” was spotted placing devices on CCTV and that a 20-year old man has been placed under armed guard with chemical burns to his arms.
Multiple news services are reporting that an eight year old child has been one of the two confirmed deaths so far.
Despite a number of Australian athletes taking part in the Boston Marathon none have been reported injured or killed in the explosions.
The US Government and FBI are treating the incident as a terrorist attack.
Image: CBS News