Thieves stole 20 phones during a single Brooklyn concert, The Post has learned — as one expert warns it’s part of a growing trend targeting crowded music venues tied to an international black market.

Cops were called to punk band Hot Mulligan’s performance at the Brooklyn Paramount on Nov. 7 when a slew of unsuspecting concertgoers were seemingly pickpocketed– with the thieves turning off the phones so they couldn’t be tracked, officials confirmed.

No arrests have been made to date, but the investigation is still ongoing, an NYPD rep said.

Attendee James Crowley, 31, said he saw multiple people scanning the floor at the Downtown Brooklyn venue, looking for phones that they thought they might have dropped.

“I’ve been going to shows since I was 14…I’ve never seen anything like that,” Cro

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