A plainclothes SFPD officer seen in the video above apparently punching a suspect as he's being arrested can then be heard saying he has to "seize" the camera that was recording the incident from a window above. The man filming it, who narrates to sensational effect (e.g. "When did North Korea win the war?"), replies to the officer, "Get a warrant."
The scene, shot at Eddy and Taylor on Friday, March 4, per the YouTube label, is perhaps a sadly typical one for the Tenderloin, and it's unclear the nature of the crime that the suspect was being arrested for. The offending punch seems to have come after the suspect tried to bite one of the officers, which you can hear him yelling during the incident. As the suspect is being cuffed and then lifted to his feet, a female bystander begins screaming some mostly unintelligible, and intelligibly nutty and homophobic remarks, apparently at the suspect. She also screams, "I killed Dan White!"
At the 3:30 mark, the plainclothes officer again addresses the cameraman in the window, demanding his name. The cameraman, smartly, replies, "That's classified," the officer proceeds to say he'll just ask others on the street.
He does know he can't do this, right?
In any event, via the Electronic Frontier Foundation, you can read about how the U.S. Court of Appeals for the First Circuit, as well as the Seventh Circuit, have in recent years unambiguously affirmed citizens' rights to videotape police conducting their duties in public. Police have tried to invoke wiretap statutes in both Massachusetts and Illinois to equate cell phone video with surreptitious audio recording, but the courts have sided with the First Amendment on this one.