The "USA Freedom Rally" was planned for 3 p.m. PT featuring multiple Republican California congressional candidates.
A group of protesters, mostly clad in black, marched from Roxbury Park to the site of the USA Freedom Rally.
"A couple of the members got into a violent attack on a pro-Trump member. It was at that point that an unlawful assembly was declared," Lt. Max Subin of BHPD told Fox News.
The pro-Trump protester had injuries to his face and was treated by the Beverly Hills Fire Department.
A Beverly Hills Courier reporter posted video of BHPD attempting to keep the Trump supporters and counterprotesters separate.
Another video posted on Twitter shows police forming a line and ordering protesters to disperse.
The Beverly Hill Police Department was already on high alert Saturday before the protests began.
"Beginning on Halloween and through Election week, the Beverly Hills Police Department (BHPD) will be on full alert throughout the residential and business districts," BHPD said in a news release Oct. 23. "Additionally, other law enforcement personnel and private security companies will offer support and coordinate closely with BHPD."
Businesses in Beverly Hills also boarded up Saturday in preparation for potential unrest, Patch reported.