Mounting tensions on US campuses boiled over on Wednesday when pro-Israel supporters attacked an encampment of pro-Palestinian protesters at UCLA, hours after police arrested activists who occupied a building at Columbia University and cleared a tent city from its campus.

Eyewitness videos from the University of California at Los Angeles, verified by Reuters, showed people wielding sticks or poles to hammer on wooden boards being used as makeshift barricades to protect the pro-Palestinian protesters before police were called to the campus.

The university canceled classes for the day on Wednesday, and UCLA Chancellor Gene Block said the school would conduct an investigation “that may lead to arrests, expulsions and dismissals.”

In a statement, Block said the “appalling” assault on pro-Palestinian demonstrators, which came hours after their encampment was declared an unlawful assembly by UCLA, was committed “by a group of instigators.”

Los Angeles Mayor Karen Bass, who returned to the city early from a trip to Washington, and California Governor Gavin Newsom each issued separate statements condemning the overnight violence and calling for an investigation.

Neither the Los Angeles Police Department nor the university answered queries from Reuters asking whether any arrests were made at the confrontation, which began around 11pm local time and went on for two or three hours.

In New York City, scores of police officers in helmets and tactical gear arrested pro-Palestinian demonstrators occupying Hamilton Hall, an academic building at Columbia University.

Undergraduate students watching the extraordinary scene, many jeering at the police, fled into nearby buildings as police also cleared out a nearby protest encampment that had inspired similar protests at campuses across the country and abroad.

Police arrested about 300 people at Columbia and City College of New York, Mayor Eric Adams said. Many of those arrested were charged with trespassing and criminal mischief.

The clashes at UCLA and in New York were part of the biggest outpouring of US student activism since the anti-racism rallies and marches of 2020.

The protests follow the Oct. 7 attack on southern Israel by Hamas militants from the Gaza Strip and the ensuing Israeli offensive on the Palestinian enclave.

Students have rallied or set up tent encampments at dozens of schools across the US in recent days, expressing opposition to Israel’s war in Gaza and demanding schools divest from companies that support Israel’s government. Many of the schools have called in police to quell the protests.

With the presidential election coming in November, Republican lawmakers have accused some university administrators of ignoring antisemitic rhetoric and harassment, and some have demanded Columbia’s President Minouche Shafik resign.

Many protesters, some of whom are Jewish, reject allegations of antisemitism. Shafik has said the protests brought rancor to life at Columbia and created a “threatening environment” for many Jewish students and faculty, while also blaming some episodes of harassment and hostile rhetoric on outsiders drawn to the busy Manhattan streets surrounding the campus.

US President Joe Biden, who has angered many protesters by funding and arming Israel, plans to give a speech on antisemitism next week at a Holocaust memorial event.

“Americans have the right to peacefully protest,” Karine Jean-Pierre, the White House spokesperson, told reporters. “Forcibly taking over a building is not peaceful.”

Republican US presidential candidate Donald Trump praised the police raid Columbia’s campus, saying it “was a beautiful thing to watch.” He called the ousted protesters “raging lunatics and Hamas sympathizers.”

UCLA protesters report violent attacks

Before the clashes in Los Angeles, UCLA officials declared that an encampment on its campus was unlawful, violated university policy and included people unaffiliated with the campus.

Afterwards, counter-demonstrators - many of them masked and some apparently older than most students - can be seen in videos throwing objects and trying to smash or pull down the wooden and steel barriers erected to shield the encampment.

Some screamed pro-Jewish comments as pro-Palestinian protesters tried to fight them off.

“I just didn’t think they would ever get to this,” said Kaia Shah, a pro-Palestinian protester and researcher at UCLA, “where our protest is met by counter-protesters who are violently hurting us, inflicting pain on us, when we are not doing anything to them.”

Demonstrators on both sides used pepper spray, and fights broke out. Pro-Palestinian demonstrators said the counter-protesters threw fireworks at them and beat them with bats and sticks.

Benjamin Kersten, a UCLA graduate student and member of the pro-Palestinian group Jewish Voice for Peace, called it “a devastating night of violence.”

“The encampment would be a peaceful effort were it not for the continuous presence of counter-protesters and agitators,” he wrote in a text message.

Police said UCLA had called them to restore order and maintain public safety within the encampment. Video later showed police clearing a central quad beside the encampment and erecting a metal crowd-control barrier in front of it.

The atmosphere was calmer on Wednesday. Hundreds of police officers were on campus and lining its perimeter. It was unclear how many arrests were made or the number of people injured.

Columbia demonstrators arrested

Columbia’s Shafik said she had asked police to stay on campus until at least May 17, two days after graduation, and the main campus, where student dorms are located, remained under lockdown on Wednesday. The school said the rest of the semester would be conducted remotely, including final exams.

“I am sorry we reached this point,” Shafik wrote in an email to the university community on Wednesday, promising efforts to reunite a frayed campus.

Ararat Sekeryan, a Slavic languages doctoral student from Istanbul, said he was pushed out of the encampment and described the police deployment as dangerous.

“I myself felt attacked,” he said. “They were so afraid of this peaceful movement that they had to send more than a thousand, maybe hundreds of police to campus.”

Ben Solomon, 22, a Jewish student at Columbia, said he welcomed the removal of what he called a “mob” from the occupied building and encampment.

The university earlier warned that students involved in the occupation faced academic expulsion.