At least two people were arrested as of 1 p.m., according to U.C. Berkeley police.
The conservative social and political commentator and writer said she still might "swing by to say hello" to her supporters as police and university officials braced for possible trouble whether she shows up or not, citing intelligence and online chatter by groups threatening to instigate violence.
As of 1 p.m., campus police have made two arrests. Both individuals’ affiliation to UC Berkeley is— UC Berkeley (@UCBerkeley) April 27, 2017
unknown at this time.
"I have my flights, so I thought I might stroll around the graveyard of the First Amendment," Coulter said in an emailed message.
Reporter Wayne Freedman with ABC7's sister station KGO-TV was on the scene when at least one protester was taken into custody. The man went peacefully, but the reason for the arrest was unknown.
Demonstrators from as far away as Orange County, some with flags that read "TRUMP," said they would stand up for free speech.
PHOTOS: Groups protest in Berkeley over Ann Coulter
Members of another group called Respect Berkeley, which was recently organized by residents, said they would provide security for protesters and defend free speech at all costs. They said they weren't showing up armed but were prepared for violence with knee pads and soapy water to wash away tear gas and pepper spray.
Oath keepers in Berkeley are filling bottles with soap water to treat skin irritated from pepper spray- just in case protest turns violent. pic.twitter.com/YNyffffR8T— Amy Hollyfield (@amyhollyfield) April 27, 2017
The tension illustrated how Berkeley has emerged as a flashpoint for extreme left and right forces amid the debate over free speech in a place where the 1960s U.S. free speech movement began before it spread to college campuses across the nation.
University police set up bright orange barricades at the university's main plaza as a precaution for possible crowd control. Berkeley police deployed dozens of officers on the Cal campus shortly before noon, and an armored police vehicle was also seen patrolling one street on campus. Also, city officers patrolled a park where two far-right groups said they would hold protests.
VIDEO: Aerial views of Berkeley demonstrations about Ann Coulter
The International Socialist Organization said it planned an "Alt Right Delete" rally about a mile from the right-wing protests and just outside the university campus to show support for free speech and to condemn the views of Coulter and her supporters.
Coulter confirmed to the Associated Press that her planned speech on illegal immigration, followed by a question-answer session, was canceled. But she remained coy about what she might do instead.
"I'm not speaking. But I'm going to be near there, so I might swing by to say hello to my supporters who have flown in from all around the country," Coulter said in an email. "I thought I might stroll around the graveyard of the First Amendment."
Officials at UC Berkeley said last week they feared renewed violence on campus if Coulter followed through with plans to speak.
They cited "very specific intelligence" of threats that could endanger Coulter and students, as Berkeley becomes a platform for extremist protesters on both sides of the political spectrum.
Efforts by the university to cancel or delay the Coulter event dealt a blow to Berkeley's image as a bastion of tolerance and free speech.
Chancellor Nicholas B. Dirks sent a letter to the campus Wednesday saying the university is committed to defending free speech but also to protecting its students.
"This is a university, not a battlefield," Dirks said in the letter. "The university has two non-negotiable commitments, one to Free Speech the other to the safety of our campus community."
Berkeley's reputation as one of the country's most liberal universities, in one of America's most liberal cities, has made it a flashpoint for the nation's political divisions in the era of Donald Trump.
Earlier this month, a bloody brawl broke out in downtown Berkeley at a pro-Trump protest that featured speeches by members of the white nationalist right. They clashed with a group of Trump critics who called themselves anti-fascists.
Similar violent clashes also erupted at the same site, a public park, on March 4.
In February, violent protesters forced the cancellation of a speech by right-wing writer Milo Yiannopoulos, who like Coulter was invited by campus Republicans.
The Berkeley College Republicans and the Young America's Foundation, a conservative group that had helped book Coulter's campus speaking events, both pulled their support Tuesday citing fears of violence. They blamed the university for failing to ensure protection of conservative speakers.
"Berkeley College Republicans do not want to endanger people's lives so because of the university's unwillingness to do their job we are forced to cancel the event," Troy Worden, president of the campus Republicans, said Wednesday.
Coulter echoed the blame on Twitter: "I'm very sad about Berkeley's cancellation, but my sadness is greater than that. It's a dark day for free speech in America."
The Associated Press has contributed to this report.