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Meek Mill had a scary end to his Halloween night -- he was detained by New York City Police Department officers ... and he says it was for no reason!

Here's the deal ... video is circulating online that shows the rapper standing with his arms behind his back, seemingly being questioned by police as supporters rally behind him early Saturday morning.

A spokesperson for the NYPD tells us police responded to reports of a male with a firearm at just after 4 AM this morning ... which led to officers stopping three men in a vehicle matching the description and license plate they received.

Cops didn't find the alleged firearm, we're told ... and, all three men were let go -- no arrests, unlike many online are claiming. X / @MeekMill

Meek spoke out about the incident on

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