DORCHESTER — Boston police say a 14-year-old walked into Harambe Park just after midnight Monday with a loaded, untraceable Sig Sauer stuffed in his waistband — and walked out in cuffs.

Officers assigned to District B-3 were running a directed patrol near 930 Blue Hill Avenue at about 12:15 a.m., sent to the area over what the department called a recent uptick in firearm violence. What they found instead was a crowd of roughly 20 people hanging out at Harambe Park after posted hours, in violation of city Parks and Recreation rules.

Then one of them broke off.

A pat frisk did the rest. Recovered from the waistband, police say, was a black Sig Sauer P250 with the serial number scratched off, 13 rounds loaded in the magazine and one already in the chamber.

The teen was booked at District B-2. He is expected to be arraigned in Dorchester District Court on seven charges: carrying a firearm without a license, possession of ammunition without a Firearm Identification Card, carrying a loaded firearm without a license, possession of a large-capacity firearm, possession of an untraceable firearm in the commission of a felony, receiving an untraceable firearm, and trespassing.

Boston police have not released the suspect's name because he is a minor.