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Two “brazen” lone gunmen opened fire at a massive Brownsville, Brooklyn, block party on Saturday night, leaving a 38-year-old man dead and 11 others injured, law enforcement sources told The Post.

The gunfire erupted just before 11 p.m. as the annual two-day “Old Timers Day” party was ending and thousands of attendees were leaving, the sources said.

The violence was preliminarily blamed on two shooters who remained at large early Sunday, and who had blasted into the crowd despite a contingent of some 100 cops monitoring the event, the sources said.

“I’ve never heard of 12 people shot in one incident — ever,” one veteran investigator told The Post of the bloodshed at Mendoza Park at Christopher and Hegeman avenues.

“This is one of the worst, most brazen shootings I’ve ever seen.”

The injured were rushed to nearby Brookdale Hospital and to Kings County Hospital.

By midnight, detectives had found one abandoned firearm and were combing the ground for more evidence among hundreds of scattered beer bottles, cans, cups and other debris.

The party is in its 56th year and features concerts, food and exhibitions.

Attendees travel from across the country to attend.

“I heard shots and I ducked,” said one 66-year-old man who was at the party.

“When I stuck my head up, and the smoke cleared, it was pure pandemonium,” said the man, who asked not to be named.

“It was chaos,” said Gary Miller, 60, a vendor at the party.

People were breaking down, having outbursts,” he said.

“We were enjoying the evening and all of a sudden shots rang out,” he remembered.

“Shots rang out and the crowd was already dispersing and you heard about 9 to 11 shots and everyone was running and scattering for cover.”

“We had a terrible shooting in Brownsville tonight that shattered a peaceful neighborhood event,” Mayor de Blasio tweeted at close to 1 a.m.

The shooting happened in the same precinct where the first of four water-dousing attacks on cops broke out on the previous Saturday.