The marathon itself, on its Facebook page, referred to the blasts as bombs. Authorities did not say immediately what caused the explosions.
Massachusetts General Hospital said it was treating 19 injured people; Tufts Medical Center reported that some injured were to be treated there as well.
The explosions sent smoke billowing into the air at Copley Square, turning a site of celebration into a mess of destruction.
Crowds had gathered in the area to watch the runners.
Out of an abundance of caution, the Lenox Hotel was evacuated, the Boston Globe reported, as authorities looked into possible security concerns.
Officials in other cities, including New York, tightened security as as result.
Mike Baingon, who works at the Atlantic Fish Company in Boston, said an explosion took place in front of the restaurant and that he was right by the front door at the time.
The explosions occurred at about 2:45 p.m., more than two hours after the first of the race's nearly 27,000 runners had crossed the finish line, CNN Producer Matt Frucci reported.
The race was halted as was subway service into the area.
Troops from the Massachusetts National Guard were assisting police as well.
Edited by AJ 12, 15 April 2013 - 03:18 PM.