Jackie Bruno, a reporter for New England Cable News, tweeted there were "horrific" injuries at the site of the blast, including people with limbs missing.
Spectators for the race, held annually on Patriots Day, were rushed to medical tents for treatment following the blasts. CNN reported 19 people have been taken to the emergency room at Massachusetts General Hospital.
WBZ reported one of the explosions happened near the viewing stands on Boylston Street, and there were multiple injured people. Smoke could be seen rising feet from the marked finish line.
Reuters stated via Twitter a spokesman for the marathon said the headquarters have been locked down. A CNN producer reported members of the crowd were injured when a pair of explosions happened, and medics are on the scene giving assistance.
Josh Cox, the U.S. 50K record holder, tweeted for people not to go near the finish line after explosions along with a picture from near the scene. The Lenox Hotel, located a block from the site, has also been evacuated.