Violence broke out at a Berkeley rally after a group of black-clad anarchists stormed the "No to Marxism" protest on Sunday.
Various fights erupted when hundreds of leftist "antifa" activists stormed the protest by right-wing demonstrators being held at the Martin Luther King Jr. Civic Center Park, the San Francisco Chronicle reported.
Trump supporters and journalists were assaulted by the counterprotesters, who were chanting "no Trump, no KKK, no fascist USA" and "Go home Nazi," the Chronicle reported, adding that 13 people were arrested.
Thousands of people gathered at the Civic Center, and Berkeley Police confirmed in a statement that at least one officer was injured during an arrest while several others were struck with paint. Additionally, six people were injured after the violence erupted at the rally.
According to the Los Angeles Times, one of their journalists was among those assaulted, as well as Mother Jones reporter Shane Bauer and Patriot Prayer conservative group leader Joey Gibson, who had opted to cancel a rally scheduled to take place the previous day amid criticism from city officials.
Police fired a rubber bullet at one demonstrator who had allegedly tried to cross through police barricades to enter the park, while other protestors set off smoke bombs, the Times said.
By mid-afternoon the crowd had dispersed, and police arrested 13 people on various charges including assault with a deadly weapon, felony assault, and violations of the Berkeley Municipal Code.
Berkeley Police Chief Andrew Greenwood said officials opted to let the anarchists enter the venue in an attempt to avoid more violence.
"The potential use of force became very problematic," he told CNBC, adding that there was "no need for a confrontation over a grass patch."
Since then police have come under fire as members of the public questioned how authorities handled the situation.