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Vic Mensa rolled up on ICE headquarters in downtown Los Angeles, stopped traffic and rocked the hell out ... and the protest show went off without a hitch.

The rapper/singer and his band, 93PUNX, stopped traffic and put on a free concert Monday afternoon in the back of a flatbed truck, parked right in front of the building.

Vic and co. broke into their single, "Camp America" ... a blistering track criticizing the U.S. government for its treatment of immigrants. Surprisingly, they were allowed to continue performing for a crowd of about 100 for nearly 40 minutes ... despite the clear "f**k ICE" theme.

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We're told cops blocked off the street before finally shutting it down. Mensa was very pleased with the outcome of his first anti-ICE show ... telling us it was a victory. Vic also reiterated the point of it all, saying the treatment of humans right now in this country is unacceptable ... so he wanted to let the people making the decisions know that.

Granted, it was Labor Day so nobody was working inside the ICE HQ ... but we're guessing the message was still received.

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