Washington (AFP) - Waco police said they had received threats after a mass brawl between rival motorcycle gangs in the Texas city left nine people dead and saw nearly 200 people arrested.

Authorities are still looking into Sunday's mayhem at the Twin Peaks Sports Bar and Grill but initial investigations suggest the huge fight involving guns, knives, clubs and chains was the result of a "turf war."

"We are only five days into a very complex investigation. We are dealing with a single incident crime scene that gave us nine deceased human beings," Waco police spokesman Patrick Swanton said in a Facebook statement.

"That is more dead than the entire count of homicides we worked in the entirety of 2014.

"We would like to remind our citizens you are safe. There have been no credible threats towards you.

"We do continue to receive threats on law enforcement but that is expected in our line of work. However, we would also like to remind those involved: you know we did nothing to start this."

Swanton rubbished claims that police had simply arrested everybody at the scene and hit out at what he called "false and ludicrous conspiracy theories" directed at the police.

"We play by a very strict set of rules and will not deviate from them to match attempts to sway the truth," he added.

"I can attest I have personally seen the horror unfold and was amazed at the actions of our heroic officers in their quest to save innocent victims' lives."

Among those arrested were bikers wearing the insignias of the Cossacks, the Scimitars and the Bandidos groups.

In the United States they are particularly associated with gun-running and drug-trafficking, and some biker clubs or their chapters have been formally designated as organized crime groups by the Federal Bureau of Investigation.