Footage of a vicious fight allegedly involving gang members and bystanders in Rotorua overnight shows people being thrown onto the road like rag dolls.

A police investigation has been launched after a bar patron was knocked unconscious and another kicked in the head as part of a violent and allegedly unprovoked gang brawl.

The Rotorua Daily Post understands Hells Angels members from Auckland were involved.

The incident took place near the Rotorua Sports Bar just before midnight and a witness says the eight-minute attack involved around 30 people.

"It was crazy, something I've never seen before," she said. "The police were very, very fast but they [gang members] had all gone by then.

"The whole lot went on for about eight minutes. It started, stopped, then started in the middle of the road and stopped, and then it started again. They kept coming from everywhere.

"At one stage there were over 30 of them fighting. They were all very very big."

Rotorua Police Detective Sergeant Caroline Wharton said along with the two seriously injured people, a number of other bystanders also sustained injuries.

No arrest has been made and Wharton called on witnesses to come forward.

A 33-second clip sent to the Rotorua Post shows a swarm of people fighting on both sides of Arawa St, in Rotorua's CBD.

While it is alleged gang members are involved, no gang patches can be clearly made out. Many involved are dressed in black clothing.

The footage shows scuffles taking place all over the road, with punches thrown and people running into one another.

Towards the end, a large person can be seen lifting another person off the ground before throwing them over their left shoulder onto the road.

Wharton said the gang-led attack was unprovoked, with a minivan load of more gang members arriving on the scene a short time after the fight started.

"This incident was frightening for those caught up in it and we are determined to hold those responsible to account," Wharton said.

"While police are following lines of inquiry, we are calling on members of the public who saw what happened, to come forward to help us in our investigation.

"Those people are asked to get in touch with police on 105, and quote file number 210523/2089."

Information can also be passed on anonymously to Crimestoppers on 0800 555 111.