SAN FRANCISCO — The University of California, Berkeley, on Wednesday canceled a scheduled speech by the conservative author Ann Coulter, in the latest blow to the institution’s legacy and reputation as a promoter and bastion of free speech.
University administrators said in a statement that they could not allow Ms. Coulter to speak because of active security threats. In a letter to the Berkeley College Republicans, which was sponsoring the speech, two university vice chancellors said the university had been “unable to find a safe and suitable venue for your planned April 27 event featuring Ann Coulter.”
The letter, written by Scott Biddy, the vice chancellor, and Stephen Sutton, the vice chancellor for student affairs, said it was “not possible to assure that the event could be held successfully — or that the safety of Ms. Coulter, the event sponsors, audience and bystanders could be adequately protected.”
After the cancellation was announced on Wednesday, Ms. Coulter posted on Twitter that “no school accepting public funds can ban free speech.”
With its reputation as one of the country’s most liberal universities, the campus and surrounding areas have become a target for small, militant and shadowy right-wing groups who in recent months have clashed with equally militant and shadowy anarchist groups based in the San Francisco Bay Area.
On Saturday, at the latest of these violent encounters, the police arrested more than 20 people. One video that went viral on social media showed a man identified as a member of a white supremacist group sucker-punching a woman who identified herself as an anarchist. These fight-club-type episodes, which have occurred both on campus and in the city of Berkeley, have escalated since the election of President Trump.
In February, a speech by the incendiary right-wing writer Milo Yiannopoulos, also sponsored by the College Republicans, was canceled after masked protesters smashed windows, set fires and pelted the police with rocks.
Dan Mogulof, a spokesman for the university, said the college regretted that it had become a magnet for militant groups. “It’s become an O.K. Corral of sorts for activists across the political spectrum,” Mr. Mogulof said.
The university, he said, was committed to having a diversity of voices on campus and was working with the police to reschedule Ms. Coulter’s appearance. “We are going to do whatever we can to make that happen at a time and a place when police can provide safety and security,” he said.
At a time of heightened polarization, Berkeley is not the only university struggling to balance free speech and security concerns. The police clashed with protesters on Tuesday outside an auditorium at Auburn University where the white nationalist leader Richard Spencer was speaking. The university had canceled the event on the grounds that it could turn violent, but a federal judge in Mongtomery, Ala., ruled that the speech should proceed because there was no evidence that Mr. Spencer advocated violence.
The episodes have become fodder for conservative critics. In February, after the cancellation of the event with Mr. Yiannopoulos, Mr. Trump posted on Twitter: “If U.C. Berkeley does not allow free speech and practices violence on innocent people with a different point of view — NO FEDERAL FUNDS?”
Both Ms. Coulter and the Young America’s Foundation, which books her college speeches, said they expected the event to proceed as planned. Spencer Brown, a spokesman for the Young America’s Foundation, which promotes conservative ideals, said in an email that Ms. Coulter’s lecture would proceed next week “whether Berkeley likes it or not.”
The violent clashes in Berkeley have presented a dilemma for the police, who say intervening has its own risks. Anarchist groups have for years appeared at protests in neighboring Oakland, punctuating peaceful demonstrations by smashing shop windows and attacking public buildings.
The Oakland police came under heavy criticism in 2011 after a protester, a former Army Ranger, was severely injured during a demonstration. The protester, Kayvan Sabeghi, said the police beat him with batons. He sued, and the City of Oakland agreed to pay $645,000 as part of a settlement.
The Berkeley campus gained national attention in 1964 as the center of a movement to expand political expression, which became known as the Free Speech Movement.