Relax — there was no loose cannon in Battery Park yesterday.

But some downtowners were jittery after two real cannons (pictured) were fired at Castle Clinton around 10 a.m. to mark the historic fort’s 200th anniversary.

‘’[The noise] was really loud. A lot of people were running. [There was] a lot of smoke,’’ said a worker at a Radio Shack on Broadway, across from the park.

The fort was built to defend the harbor prior to the War of 1812. It was dedicated on Nov. 25, 1811.

Members of 7th Army Corps Artillery, wearing period costumes, fired 13 blanks from the cannons to mark the date.

One person called 911 from a building at nearby 11 Broadway to report a possible explosion.

Manny Rosa, a nearby bank security guard, said a female passerby asked him what happened and said “she thought maybe a barge had crashed into the dock.’’

Others took it in stride and were just curious.

“I was approached by a number of tourists who asked me, ‘What was that?’ ’’ said one police officer.

Police said they had been notified in advance of the event.