Previous protests around the convention have been largely peaceful.

"There are people that their only reason for being here is disruption, disorder, chaos, hurting people and damaging property," Williams said. "Those are the only people at this point that are giving us any kind of trouble at all."

Trump, a wealthy New York real estate developer, has drawn the ire of many Americans with proposals that include building a wall along the US-Mexico border and restricting immigration from Muslim-dominated countries. He was nominated on Tuesday evening.

Police have confiscated items including gas masks, tent stakes and metal screws inside the secured zone around the convention site, where police are restricting such items, Williams said.

"Almost every prohibited item we have confiscated," Williams said.