In Berkeley Saturday morning, and into the afternoon, hundreds of demonstrators on opposite ends of the political spectrum once again clashed in the city's Civic Center Park, breaking into fights, lighting fireworks, and generally causing mayhem. As NBC Bay Area reports, the crowds — both attendees of a "Patriot's Day" rally and a counter-protest, and inevitably others who show up to these things just to participate in anarchic behavior — spilled out over to Shattuck Avenue in downtown Berkeley, and police were advising people to avoid the area.

As of noon or just after, police said they had made 15 arrests and anticipated more.

Also, Berkeley cops have brought in Alameda County Sheriff's deputies for backup, and NBC Bay Area reports that "rocks and bottles were launched at officers and many were assaulted."

ABC 7 reports that "some type of gas" was released in the park this morning, and thus officers have had to wear gas masks."

In response to the "civil disturbance," BART was not making stops at Downtown Berkeley station, and that remained the case as of 4 p.m.

Giving a speech at one more secured end of the park was alt-right blogger Lauren Southern, as the LA Times reports, railing against "societal change, Kim Kardashian and the media," and calling on the alt-right to become more like the left in their tactics, "subversive."

This is the third time in recent months that a clash like this has taken place in the streets of Berkeley, beginning with the night of a planned Milo Yiannopoulos speech on the Cal campus on February 1, and continuing with a rally and counter-protest similar to the ones held today just a month ago, on March 4. Berkeley police continue to seek suspects who were caught on camera assaulting others during that event.

CNN has picked up the story, noting that the violent clash in Berkeley is just one of many demonstrations happening around the country for Tax Day, with some of these focused on pushing for President Trump to release his tax returns.

TMZ had some helicopter footage of one mass fight breaking out in Berkeley where demonstrators can be seen wielding sticks and carrying shields.

In one video below, you can see where demonstrators on either side of a makeshift orange barrier end up crossing over it to engage in skirmishes with each other. And one demonstrator is carrying a a sign with that symbol of the alt-right, Pepe the frog, and the message "Green Lives Matter."

Meanwhile, a more peaceful Tax Day rally was going on in San Francisco's Civic Center, with Jane Kim and Nancy Pelosi making appearances, among others.

Previously: Berkeley Braces For Another Pro-Trump Rally Saturday, Cancels Farmers Market