The participants would have been in the area but the race had been delayed because of an unattended bag.
The charity race, in its third year, is in aid of Marines and sailors and this year attracted thousands of runners.
The blast occurred at about 09:30 local time (13:30 GMT) on a boardwalk area, which authorities sealed off.
State and federal law enforcement agencies are investigating the explosion and are sweeping the area for more devices. They urged people to stay away.
NBC later reported that a second device had been found.
Ocean County sheriffs later said that 30 homes near the site of the explosion were being evacuated.
Three people were killed and 260 injured when bombs exploded at the finishing line of the Boston Marathon in 2013.
More about:
