In what has become an eerily familiar scene in Berkeley, supporters and detractors of President Donald Trump on Saturday exchanged blows, burned flags, and set off fireworks at opposing rallies at which 20 people were arrested and 11 injured, police said.

The political divisiveness reflected across the United States took the form of a makeshift barrier — comprising net and traffic cones — erected between the two equally impassioned factions. The barrier, however, failed to keep the skirmishes at bay.

About 200 people gathered at Martin Luther King Jr. Civic Center Park around 10 a.m. on Tax Day as did Berkeley police officers — in riot gear. Police estimate that the crowd swelled to between 300 and 700 people before it dispersed around 4 p.m.

Nearly two dozen people were arrested and roughly one dozen were injured following violent protests in Berkeley. Thom Jensen reports.

Early in the day, though, attendees of the "Patriot's Day" rally and Antifa-organized counter protest exchanged heated words and several people started pushing each other. Police quickly arrested one man. Others were arrested as several fistfights broke out. Some of those arrested face charges related to assault and assault with a deadly weapon.

Of those who were hurt amid the bedlam, one person was stabbed and seven were taken to nearby hospitals, according to Berkeley police.

Social media posts indicated that Kyle Chapman aka Based StickMan or Alt-Knight was among those who got arrested. The 41-year-old from San Francisco was also taken into custody at a "March 4 Trump" last month for bashing an Antifa protester in the head with a stick. It's unclear what prompted his arrest Saturday.

[BAY RM] Violence Erupts at Opposing Rallies in Berkeley

Social media posts from the chaotic scenes in Berkeley described a "small blast," the use of tear gas and pepper spray, property damage and fireworks whizzing through crowds. Photographs showed a man covering his eyes as blood covered his hands and dripped down his face.

Trump supporter John Beavers from Washington state got smacked in the face.

"We’re over there, shaking a hand, and a bomb went off and they want to hit me," he said.

People were clad in "Make America Great Again" sweatshirts and flags, and others were dressed in black with their faces hidden behind helmets and masks, according to footage from the scene. Flags read "Anti-fascist anti-racist," "Defend America," "Free speech" and "Fascist scum your time is done."

In what has become an eerily familiar scene in Berkeley, supporters and detractors of President Donald Trump on Saturday exchanged blows, burned flags, set off fireworks, and were arrested at heated opposing rallies. Christie Smith reports.

"These Trump people are here because they want to start things," Jeff Vernon of Berkeley said.

But Stephanie Edd, also of Berkeley, viewed the protest as an attempt "to co-opt the history of free speech."

By midday, Allston Way between Milvia Street and Martin Luther King Jr. Way was blocked, as protesters moved from the park to the streets, followed closely by officers. Police urged residents to avoid the area of Center and Miliva streets. Soon the warning extended to include Shattuck Avenue and Center Street.

NBC Bay Area's SkyRanger captured people burning flags — with one man posing for a selfie beside the torched cloth — and others punching each other and using helmets and skateboards as weapons. Demonstrators were also seen tipping over a garbage can and igniting its contents.

An investigation into Saturday's violence is ongoing. Police are asking the public to submit photos and video of the protests to help them identify additional suspects.

Protesters pushed a trash bin back and forth during a fight that broke out during opposing rallies Saturday in Berkeley. At least 13 people were arrested.

In order to prevent a bloody riot, city leaders on Friday had announced that pipes, bats, bricks, rocks, glass bottles, shields, pepper spray, sticks, and other potentially dangerous items were not allowed at the park during the rallies.

Toward that end, police checked backpacks before allowing them into the park. Even so, they tweeted that officers had confiscated a knife, a stun gun, flagpoles, and other contraband.

Berkeley police were forced to call in Alameda County sheriff's deputies for backup as rocks and bottles were launched at officers and many were assaulted, according to Twitter.

The downtown Berkeley BART station was also closed.

A weekly farmers market at the Berkeley park was called off amid the scheduled political protests, according to reports.

Saturday's rallies follow a "March 4 Trump" rally and violent protests when controversial speaker Milo Yiannopoulos came to the UC Berkeley campus.

Editor's note: Berkeley police initially reported 15 arrests, but later updated that number to 13.