Brisbane police are investigating after one of their officers was filmed appearing to punch and knee a man while he was being held by another officer.

The video, which was filmed by Jacob Grady during an arrest in Fortitude Valley on the weekend and posted to social media, and is believed to have begun after the man was ejected from a nearby venue.

"They had about eight police officers on the one guy, with the blows that the guy was taking he was just getting pulverised," Mr Grady said. "At the start I was worried about the police officers but by the end I was worried about the guy and about how badly hurt he would get."

"In the end his face was on the ground and there was so much blood on his face."

View photos Jacob Grady filmed the incident. Picture: 7 News More

The video shows one officer punching the man in front of a large crowd before another officer runs in and charges him into a wall. The original officer then throws a number of knees at the man.

View photos

View photos

The police union has defended the video, saying it was just a few seconds in a much larger incident while Queensland Police have confirmed they are investigating the arrest.

"The Queensland Police Service will examine the video and all circumstances leading up to the incident depicted," a statement read.

"Any member of the the public who has a complaint about the conduct of police is encouraged to contact the QPS on 131 144 or the Crime and Corruption Commission."