Police were ramping up security in Waco, Texas, Monday, fearing further violence in the wake of a brawl and shootout between members of rival motorcycle gangs a day earlier that left nine dead and 18 injured.

Authorities increased security to quell fresh attempts at criminal activity in the Central Texas town following the melee Sunday, Waco Police Sgt. W. Patrick Swanton said.

"Our citizens are safe. I will tell you that we have had threats against law enforcement officers throughout the night from various biker groups. We are very aware that some of them have come into our city and we have a contingency plan to deal with those individuals if they try to cause trouble here," Swanton said at a news conference early Monday.

Parts of downtown Waco were on lockdown after the shooting, and officials stopped and questioned motorcycle riders. Agents from the FBI and federal Bureau of Alcohol, Tobacco, Firearms and Explosives were assisting local and state authorities in the investigation.

About 170 people were arrested on charges of engaging in organized crime. Earlier, Swanton said 192 people had been arrested but later revised that number downward.

The violence erupted shortly after noon local time at a busy shopping center along Interstate 35 that was full of people, including families marking Baylor University's graduation this weekend. Authorities said the gangs were meeting at a Twin Peaks restaurant to settle disputes over territory and recruitment.