Police arrested two teens accused in a string of armed robberies after an off-duty detective saw them putting on masks on a Merrick Avenue sidewalk.

Authorities say 19-year-old Jahmani Harleston, of Brentwood, and 18-year-old D'Andre Dogostiano, of Rockville Centre, tried to rob a Westbury 7-Eleven at gunpoint around 3 a.m. Tuesday.

They allegedly pistol-whipped the clerk, knocking him to the floor, then fled without taking any money.

Then the off-duty detective spotted them before they allegedly tried to strike the Fast Break Deli in North Merrick. He called 911, the suspects fled, and police say they responded, searched the area and caught both men hiding in a truck down the block.

As officers led Dogostiano out of Nassau Police Headquarters, he proclaimed his innocence in a profanity-laced denial. He also said Harleston was innocent.

But according to police, Harleston was also involved in four other robberies in Nassau within the past 10 days.