The university blamed "150 masked agitators" for the unrest, saying they had come to campus to disturb an otherwise peaceful protest.
Administrators decided to cancel the event about two hours before the Breitbart editor's speech. UC Berkeley said it removed him from campus "amid the violence and destruction of property and out of concern for public safety."
Black-clad protesters wearing masks threw commercial-grade fireworks and rocks at police. Some even hurled Molotov cocktails that ignited fires. They also smashed windows of the student union center on the Berkeley campus where the Yiannopoulos event was to be held.
At least six people were injured. Some were attacked by the agitators -- who are a part of an anarchist group known as the "Black Bloc" that has been causing problems in Oakland for years, said Dan Mogulof, UC Berkeley spokesman.
More than 1,500 protesters had gathered at Sproul Plaza, chanting and holding signs that read: "No safe space for racists" and "This is war."
The violent protesters tore down metal barriers, set fires near the campus bookstore and damaged the construction site of a new dorm. One woman wearing a red Trump hat was pepper sprayed in the face while being interviewed by CNN affiliate KGO
She was able to respond that she was OK after the
As the scene spiraled out of control, university police warned protesters to disperse and issued a lockdown for campus buildings.
"We condemn in the strongest possible terms the violence and unlawful behavior that was on display and deeply regret that those tactics will now overshadow the efforts to engage in legitimate and lawful protest against the performer's presence and perspectives," UC Berkeley said in a statement.
"While Yiannopoulos' views, tactics and rhetoric are profoundly contrary to our own, we are bound by the Constitution, the law, our values and the campus's Principles of Community to enable free expression across the full spectrum of opinion and perspective," it stated.
As police dispersed the crowd from campus, a remaining group of protesters moved into downtown Berkeley and smashed windows at several local banks.
No arrests were made throughout the night.
Yiannopoulos on college speaking tour
Yiannopoulos had been invited to speak by the Berkeley College Republicans. He has been on a college speaking tour and had planned to speak about cultural appropriation on Wednesday.
In a Facebook Live video, Yiannopoulos described what happened as "an expression of political violence."
"I'm just stunned that hundreds of people ... were so threatened by the idea that a conservative speaker might be persuasive, interesting, funny and might take some people with him, they have to shut it down at all costs."
President Donald Trump weighed in on Twitter, suggesting the possibility of cutting federal funds to the public university.
"If U.C. Berkeley does not allow free speech and practices violence on innocent people with a different point of view -- NO FEDERAL FUNDS?" Trump tweeted Thursday morning.
But some protesters said the Yiannopoulos event wasn't a matter of free speech, because he espouses hate speech.
UC Berkeley said it had prepared security measures following what had happened at Yiannopoulos' previous events. One of his planned speaking engagements at UC Davis was also canceled last month in response to protests.
"Ultimately, and unfortunately, however, it was impossible to maintain order given the level of threat, disruption and organized violence," UC Berkeley said in a statement.
Most UC Berkeley students who spoke with CNN said they were relieved that Yiannopoulos wasn't able to speak, but this was not how they wanted to accomplish that goal.
One student told CNN that he didn't agree with what happened.
"It's a sad irony in the fact that the Free Speech Movement was founded here and tonight, someone's free speech got shut down. It might have been hateful speech, but it's still his right to speak," said Shivam Patel, a freshmen who stood outside of Sproul Plaza.
The Free Speech Movement started at UC Berkeley in 1964 after students protested en masse when administrators tried to restrict their political activities on campus.
Patel said he supported peaceful protests, but disagreed with the way things turned out on Wednesday.
"It allows people on the right to say, 'Look at all these liberal Berkeley snowflakes. They're intolerant of speech.' I don't think it's productive at all. It does nothing to help this country."