BERKELEY (CBS SF) — Berkeley’s police chief defended the crowd control tactics his officers used during Saturday’s violent clash at a Patriot’s Day rally that led to 20 arrests and 11 injuries.

In a memo to Berkeley Mayor Jesse Arreguin and the City Council, Chief Andrew Greenwood said officers “found that once again large elements of the factions arrived in the area armed and prepared to fight.”

He wrote that police confiscated dozens of weapons, including sticks, wooden dowels, poles, a stun gun, knives, bear spray, an axe handle and pepper spray.

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Greenwood said officers had erected temporary fencing in the park and were trying to keep the two angry factions apart.

“At one point, about three dozen ‘Antifa’ entered the park over fencing, ultimately confronting demonstrators, and escalating violence, with fights and assaults breaking out,” he said.

Among the violent acts captured on video was a punch thrown by Nathan Damigo – a known white supremacist who attends California State University, Stanislaus – to the face of Antifa protester Louise Rosealma. She also was slugged by at least two other demonstrators.

During the melee, Rosealma said she never saw any Berkeley police officers.

“A fight within a volatile crowd is not a simple matter in which to intervene,” the chief said in the memo. “Intervening on intermixed groups of armed participants fighting or eager to fight presents challenges. Intervention requires a major commitment of resources, a significant use of force, and carries

with it the strong likelihood of harming those who are not committing a crime.”

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In his memo, Greenwood said officers were “rightly expected to not get swept into the volatility of the crowd.”

“Our responsibility in this situation is to act with deliberation, and keen awareness of context, of what actions we’re taking and why, and of what effect or reaction our actions may generate,” he wrote.

“A fight within a volatile crowd is not a simple matter in which to intervene. Intervening on intermixed croups of armed participants fighting or eager to fight presents challenges,” he added.

Several hundred demonstrators and counter-demonstrators gathered at the Martin Luther King. Jr. Civic Center Park for the rally on Saturday. A similar rally was held at the park on March 4 and resulted in 10 people being arrested.

He said of the 20 arrested Saturday, two were suspects the Berkeley police have identified as as being part of March 4 violence.

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Greenwood said at least eight officers were injured including several officers who reported hearing loss caused by having illegal explosives thrown at them, exposure to pepper spray and a knee injury.

Twenty people were arrested at the rally more suspects were expected to be detained after authorities examine surveillance photos.

“Investigations into several felonious assaults is focusing on identifying and apprehending the suspects in these cases,” he said.

Greenwood said there were “multiple assaults between elements of the crowd” and people on both sides “actively sought confrontations with those with opposing views.”

The chief said that although his department had asked people to separate themselves from people who commit violence, “crowds of onlookers and video recorders were suffused throughout the event, frequently placing themselves in very close proximity to those who were fighting.”

Greenwood said Berkeley police requested mutual aid “after our resources were essentially completely deployed” and the Oakland Police Department responded with 180 personnel, including squads of officers, motorcycle officers, supervisors and commanders.