WASHINGTON — US President Joe Biden has urged pro-Palestinian protesters on university campuses to uphold the rule of law.

"We are a civil society, and order must prevail," Biden said from the White House, in his first direct remarks about a wave of student unrest.

Police have detained more than 2,000 people nationwide in the past fortnight at college rallies and protest camps.

That includes 209 arrests early on Thursday at the University of California, Los Angeles (UCLA).

Hundreds of officers in riot gear moved on to UCLA's main campus before dawn and cleared its pro-Palestinian camp.

They set off flash bangs and flares, loaded demonstrators on to police buses, and tore down the makeshift barriers and tents that had been erected on campus a week ago.

In a statement, UCLA called the camp "both unlawful and a breach of policy. It led to unsafe conditions on our campus and it damaged our ability to carry out our mission."

"Demonstrators directly interfered with instruction by blocking students' pathways to classrooms," it added, while their clashes with pro-Israeli counter-demonstrators "put too many [students] in harm's way".

Addressing the nationwide protests hours later, Biden said: "We are not an authoritarian nation where we silence people or squash dissent. But neither are we a lawless country.

"There's the right to protest but not the right to cause chaos. People have the right to get an education, the right to get a degree, the right to walk across the campus safely without fear of being attacked."

The Uncommitted National Movement, a group of Arab-American voters opposed to Biden's re-election campaign this year, accused him of "smearing" anti-war protesters.

"It's clear Biden isn't listening to young people nationwide, or to the over half a million uncommitted voters asking him to change course. We hope he hears us before it's too late," leader Abbas Alawieh said.

The campus protests in support of Gaza have now spread to nearly 140 colleges in at least 45 states, according to a BBC tally, and at least six other countries.

Demonstrators, who have been calling for a ceasefire in the Gaza war, are also demanding academic institutions financially divest from Israel and companies who stand to make money from the conflict.

But many colleges have called in the police, with violence erupting on some campuses, as well as rising reports of antisemitic harassment against Jewish students.

The tensions at UCLA's main Westwood campus erupted on Tuesday night when a masked pro-Israeli group breached the tent camp on Dickson Plaza and attacked campers with bats, tear gas and other items.

Police appeared to move slowly and were criticised by students, observers and some political leaders for their response.

After regaining control of the area on Wednesday, law enforcement created a tight security cordon.

Administrators had already declared "an unlawful assembly" and, early on Thursday morning, officers moved in.

As police breached the perimeter, some demonstrators could be seen locking arms and using their plywood barriers as makeshift shields, while others donned hard hats and seemed to arm themselves with umbrellas.

Karen Bass, the mayor of Los Angeles, said she had been at an "incident command post" alongside law enforcement leaders at the campus throughout the operation.

"Harassment, vandalism and violence have no place at UCLA or anywhere in our city," she wrote in a statement.

Kenza, a UCLA student involved with the protests (she did not provide her last name), told the BBC the encampment had been "completely peaceful".

"It's absolutely ridiculous that we would be deemed as a threat to civil society when the reality is we have been harassed for the past week," she said.

Campus operations will be limited on Thursday and Friday, with all classes moving to remote instruction, UCLA said.

A handful of institutions — including Northwestern, Brown, the University of Minnesota and the University of Vermont — have reached agreements with protesters over divestment matters.

But arrests have been made in the past 24 hours at Yale, Dartmouth, Stony Brook, Portland State, the University of Wisconsin and the University of Texas at Dallas. — BBC