At Columbia and at the City University of New York, where police cleared demonstrators out overnight, some students decried "rough and aggressive" tactics used by officers. AFP

Police tore down a protest encampment at the University of Texas on Wednesday, arresting more than a dozen people, as unrest over Israel's war against Hamas in Gaza simmered on US campuses.

Officers also detained several people at Fordham University in New York and cleared an encampment set up inside a school building, officials said, and law enforcement were on standby at Columbia University across town after mass arrests the previous evening.

At the Massachusetts Institute of Technology protesters dug in, blocking an avenue near the center of the campus in Cambridge during the height of Wednesday afternoon's rush hour commute.

And dozens of police cars patrolled at the University of California, Los Angeles in response to violent clashes overnight when counter-protesters attacked an encampment of pro-Palestinian students.

The University of Texas Dallas saw police remove an encampment and arrest at least 17 people for "criminal trespass," the school said.

Demonstrators have gathered in at least 30 US universities since last month, often erecting tent encampments to protest the soaring death toll in the Gaza Strip.

But the sight of helmeted officers at two of America's most prestigious universities left some students dismayed.

"I don't think we should have a heavy police force on campus," UCLA student Mark Torre, 22, told AFP as he surveyed the scene from behind metal barriers.

"But more and more, day by day, I think it's a necessary evil, to at least keep safety on campus."

At Columbia and at the City University of New York, where police cleared out demonstrators overnight, some students decried the police behavior.

"We were assaulted, brutally arrested. And I was held for up to six hours before being released, pretty banged up, got stomped on, got cut up," one CUNY student who gave his name only as Jose told AFP.

A medical student offering treatment to detainees as they were released described a litany of injuries.

"We've seen things like severe head traumas, concussions, someone was knocked unconscious in the encampment by police, someone was thrown down the stairs," said the student, who gave her name as Isabel.

About 300 arrests were made at Columbia and CUNY, Police Commissioner Edward Caban said.

Mayor Eric Adams blamed "outside agitators" for ratcheting up tensions. Columbia students have denied outsiders were involved.

University president Minouche Shafik, who has come under fire over her decision to call in police, said the turn of events "filled me with deep sadness."

"I am sorry we reached this point," she said in a statement.

The protests have posed a challenge to university administrators trying to balance free speech rights with complaints of criminal activity, anti-Semitism and hate speech.

The administration of President Joe Biden -- whose support for Israel has outraged many protesters -- has also tried to walk that line.

"We believe it's a small number of students who are causing this disruption, and if they're going to protest, Americans have the right to do it in a peaceful way within the law," White House Press Secretary Karine Jean-Pierre told reporters.

Biden's rival in the November election, Donald Trump, voiced his full-throated support for the police response at Columbia.

"It was a beautiful thing to watch. New York's finest," he told a rally in Wisconsin.

"To every college president, I say remove the encampments immediately, vanquish the radicals and take back our campuses for all of the normal students."

Late Tuesday, police entered Columbia's campus and climbed into Hamilton Hall -- barricaded by protesters -- via a second-floor window before leading out people in handcuffs. They also cleared the large tent encampment.

In Los Angeles, counter-protesters sprayed chemical substances onto the pro-Palestinian encampment and attempted to tear down wooden boards and metal barricades before police eventually arrived.

On Wednesday, students on loudspeakers called for demonstrators to keep going at a camp blocking the entrance to one of the school's main libraries, which bore graffiti reading: "Free Gaza."

Elsewhere, police moved in at the University of Wisconsin at Madison and arrested several protesters, TV footage showed.

At the University of Arizona, police said they used "chemical irritant munitions" to disperse "an unlawful assembly."

The militants also took about 250 hostages.

Israel's retaliatory offensive has killed more than 34,500 people in Gaza, mostly women and children, according to the Hamas-run territory's health ministry.