Berkeley, Calif., police arrested 20 people Sunday and confiscated "dozens of weapons" as members of so-called "alt-right" and far-left groups gathered in the city for opposing protests.

In addition to the arrests, officials said three people suffered minor injuries after a group of "extremists" threw "explosives" -- believed to be fireworks and flares -- at police and Alameda County Sheriff's officers. No members of law enforcement were hurt.

Berkeley police also said that "an extremist element among a large group" damaged 21 city vehicles, setting one on fire, and slashed their tires. The group also set fires in trash bins, which were extinguished quickly.

Two groups of alt-right protesters had announced plans for a "No to Marxism" rally at the city's Civic Center Park earlier in the week. That prompted plans for a "Sweep Out The Fascists" march and counter-rally that drew hundreds of people downtown.

The San Francisco Chronicle reported that several dozen "anti-Marxist" protesters arrived at Civic Center at 12:30 p.m. At one point, the group pushed books off a table set up by a local Communist bookstore, but police and bystanders stepped in to head off any potential confrontation.

City officials said neither group had sought or obtained a permit for their respective rallies. On Friday, police issued rules prohibiting masks and items that could be used as weapons.

The police department posted photos of weapons they had seized on Facebook and Twitter. They included a hammer, rocks, wooden sticks, metal rods, and canisters of pepper spray.

“We are confiscating weapons and making arrests,” police said.

Sixteen of the arrested protesters had been identified as of Sunday evening. They included Jason Wallach, 41, and Kate Brenner, 69, both of Oakland, who were charged with possession of a banned weapon, police said. Another Oakland native, 35-year-old Andres Gonzalez, was charged with five counts of carrying a banned weapon. Jeffrey Garten, 28, of Oakland was charged with a single count of carrying a banned weapon.

Kristen Edith Koster, 50, of Berkeley, was charged with possession of a dangerous weapon. Maria Lewis, 29, of Emeryville, was charged with carrying a banned weapon and working with others to commit a crime.

Thomas Parker, 22, of Berkeley, was also charged with working with others to commit a crime, as was 27-year-old Caitlin Boyle of Oakland. Blake Griffith, 29, of Oakland, was charged with vandalism.

Sarena Perez, 39, of Oakland, was charged with possession of a banned weapon. David Chou, 26, of Santa Cruz, was charged with possession of a banned weapon and working with others to commit a crime. Freddy Martinez, 31, of Berkeley, was charged with battery.

Ericka Sokolower-Shain, 28, and Javier Cruz-O'Connell, 22, both of Berkeley, were charged with possession of a banned weapon. Jamie Hill, 30, of Emeryville, was also charged with possession of a banned weapon, as was 27-year-old Bella Podolsky of San Francisco.

“We’ve seen what the patterns are and one of the patterns is using nonviolent protests as a cover for violence and what we encourage people to do is to separate themselves from violence,” Berkeley city spokesman Matthai Chakko told Fox station KTVU on Saturday.

Events last year turned violent when so-called “alt-right” groups clashed with anti-fascist groups, the station reported.

5 ASSAULTED AT BERKELEY PROTESTS AS BLACK-CLAD ANARCHISTS STORM RIGHTWING RALLY

“No to Marxism” rally organizer Amber Cummings held a “No to Marxism” rally in Berkeley last August in which 10 people were arrested, the Bay City News Service reported. A person dressed in black, the hallmark of the infamous Black Bloc, which advocates violence, destroyed another person’s camera, according to the news outlet.

Police said the list of banned items at Civic Center and Ohlone parks included metal pipes, baseball bats, glass bottles, pepper spray, knives or daggers, shields and slingshots, KTVU reported.

Police also banned people from wearing masks, scarves or bandannas or anything covering their faces except for religious or medical reasons.

Sunday's protests came one day after police in Portland, Ore. clashed with demonstrators protesting a rally by far-right groups. Local media reported that police ordered the counter-protesters to disperse, then moved in behind a volley of stun grenades. One of the rounds reportedly hit a counter-protester in the head, becoming embedded in his helmet and injuring him. One woman was taken to a hospital after being hit in the arm and chest with a "flash-bang" grenade.

Four people were arrested.

On Sunday, Portland Police Chief Danielle Outlaw ordered a review of the use of force by officers to determine if they followed policy and training guidelines.

Fox News' Samuel Chamberlain and The Associated Press contributed to this report.