WACO, Texas - A shootout among rival biker gangs at a popular Texas restaurant left nine people dead and 18 others injured, a police spokesman said on Sunday, sending panicked patrons and bystanders fleeing for safety.

The violence erupted shortly after noon at a busy Waco marketplace along Interstate 35 that draws a large lunchtime crowd. Waco police Sgt. W. Patrick Swanton said eight people died at the scene of the shooting at Twin Peaks restaurant and another person at a hospital. He told the Waco Tribune-Herald that the nine killed were all members of biker gangs.

Another 18 people were taken to hospitals with injuries that include stab and gunshot wounds. Some victims are being treated for both, he said.

"There are still bodies on the scene of the parking lot at Twin Peaks," he said. "There are bodies that are scattered throughout the parking lot of the next adjoining business."

A photo from the scene showed dozens of motorcycles parked in a lot. Among the bikes, at least three people wearing what appeared to be biker jackets were on the ground, two on their backs and one face down. Police were standing a few feet away in a group. Several other people also wearing biker jackets were standing or sitting nearby.

Swanton said police were aware in advance that at least three rival gangs would be gathering at the restaurant and at least 12 Waco officers in addition to state troopers were at the restaurant when the fight began.

When the shooting began in the restaurant and then continued outside, armed bikers were shot by officers, Swanton said, adding that the actions of law enforcement prevented further deaths. It was not known if any of the nine dead were killed by police officers.

It's not known what triggered the violence but Swanton and McLennan County District Attorney Abel Reyna said tensions had simmered among rival gangs for months.

"Apparently the management (of Twin Peaks) wanted them here and so we didn't have any say-so on whether they could be here or not," Swanton said.

Attempts to contact Twin Peaks for comment were not immediately successful.

The fight inside the restaurant began when punches were thrown and it quickly escalated to include chains and knives before firearms were drawn, Swanton said.

Three armed gang members were arrested when they attempted to enter the shooting scene to retaliate against rival members, he said. Authorities are increasing security in the area to prevent further violence among the gangs.

In addition to local and state police, the federal Alcohol, Tobacco, Firearms and Explosives (ATF) was also on the scene, according to its official twitter account.