US President Joe Biden insisted Thursday that "order must prevail" on college campuses after weeks of turmoil, clashes with police and mass arrests involving student protests against Israel's war with Hamas in Gaza.

Biden, who until now had remained tight-lipped on the student unrest, spoke just hours after hundreds of police moved in to forcibly clear a sprawling encampment at the University of California, Los Angeles, tearing down barriers and detaining dozens of protestors.

For weeks, authorities on campuses from New York to California have tried to thread the needle between the right to protest and complaints of violence and hate speech, resulting in hundreds of arrests and chaos as university terms end.

"We are not an authoritarian nation where we silence people or squash dissent," Biden, who has faced criticism from all sides of the political spectrum over the demonstrations, said in a televised statement from the White House.

"But neither are we a lawless country. We're a civil society, and order must prevail," he added.

Earlier, UCLA students clad in white helmets linked arms and formed a line facing off against officers, who were detaining protesters and leading them away.

Police used flashbangs to disperse the crowds gathered outside the encampment who chanted "Let them go!" as helicopters hovered overhead.

Officers blocked stairs accessing the site, with students dressed in yellow jackets and serving as medics telling AFP they were being largely prevented from reaching the area.

In another part of the encampment, students carrying umbrellas, helmets and plastic shields squared off against police in mostly tense silence, with sporadic chants of "Free Palestine!" and "From the river to the sea, Palestine will be free!"

The large police presence, including LAPD and California Highway Patrol officers, came after law enforcement were criticized for being slow to act during violent clashes late Tuesday when counter-protesters attacked an encampment of pro-Palestinian students.

UCLA said classes would be remote on Thursday and Friday due to the "emergency on campus," and warned students to avoid the protest area.