A video of cops in Baltimore manhandling a guy for filming them while they arrested people is getting some attention online.

It happened late Saturday night, and from the looks of it, the cops ONLY did it because the guy was recording.  Meanwhile, he was respectful the entire time, and just kept asking if it was illegal to record them.

The big line is when one of the cops tells him "don't open your mouth," and he says, "I thought I had freedom of speech."  Then the cop responds by saying, "You DON'T."

Officials say they're launching an investigation.

He gets confronted at :19, and :31. But the worst part starts at 1:10.  WARNING:  Profanity, including the F-word.

Baltimore County cops doing their thing at the Towson bars. from Gootz on Vimeo.