(AP) -- A pipe bomb exploded in a Jersey Shore town Saturday shortly before thousands of runners were due to participate in a charity 5K race to benefit Marines and sailors, authorities said.

No injuries were reported in the blast in Seaside Park around 9:30 a.m. Saturday, said Al Della Fave, spokesman for the Ocean County Prosecutor's office. He also said no surrounding structures suffered any apparent damage.

The third annual Semper Five run was canceled, and the immediate area was put on lockdown.

The pipe bomb was in a plastic garbage can when it exploded, Della Fave said. Authorities immediately cleared the boardwalk and beach area, and bomb-sniffing dogs were used to search the area.

Authorities said there was a report of at least one other explosive device that a New Jersey State Police bomb team would "make safe," but further details were not disclosed.

Officials said runners had been scheduled to pass by the area around the time the blast occurred. But the start of the race had been delayed after an unattended backpack was found, and the area was mostly empty when the blast occurred.

Numerous state and federal law enforcement agencies are investigating.

It wasn't clear when the boardwalk and beach will reopen. Some homes near the scene were evacuated as a precaution, and the public was being urged to avoid the area.

The group that organized the event thanked Seaside Park police and the other emergency responders "for their quick action" following the blast. It wasn't immediately clear if the charity race will be rescheduled.