Police on Wednesday took back a lecture hall from pro-Palestinian protesters who for hours occupied the building at the University of California, Irvine, then cleared a student encampment that stood for more than two weeks, witnesses said.

Officers from about 10 nearby law-enforcement agencies converged on the campus after university officials requested help because protesters had occupied the lecture hall, leading the school to declare it a "violent protest," police and university officials said.

About four hours later, police had ejected the protesters from both the lecture hall and the plaza that had been the site of the encampment, according to the university and Reuters witnesses.

"The police have retaken the lecture hall," UC Irvine spokesperson Tom Vasich said by telephone from the scene. "The plaza has been cleared by law-enforcement officers."

Vasich said there were a "minimal number of arrests" and characterised the protesters as "begrudgingly cooperative."

The university said all classes would be held remotely on Thursday, asking employees not to come to campus.

The demonstration at Irvine, about 65km south of Los Angeles, is the latest in a series of campus protests across the United States over the war in Gaza in which activists have called for a ceasefire and the protection of civilian lives while demanding universities divest from Israeli interests.

UC Irvine protesters had established an encampment adjacent to the lecture hall on April 29 similar to those at other universities that have led to mass arrests and clashes with police elsewhere in the country.

On Wednesday, 200 to 300 protesters took over the lecture hall at a time when no classes were in session, Vasich said.

Police responded in riot gear and formed a barricade while an officer on a loudspeaker warned the crowd that they had formed an unlawful assembly and risked arrest if they remained, the Orange County Register reported.

Students chanted slogans, banged drums and hoisted banners, with rows of police standing nearby, Reuters witnessed.

One banner hung from the building declared the site "Alex Odeh Hall," in honor of a Palestinian activist who was killed in a 1985 office bombing in the nearby city of Santa Ana.

Four adjacent research buildings with potentially hundreds of people inside were locked down, and those inside were instructed to shelter in place, Vasich said, though the university later altered that instruction and instead advised them to leave.

Shortly before nightfall, police moved in on the lecture hall, then engaged in a tense standoff with protesters at the encampment.

Helmeted police wielding batons formed a line against protesters. Police gradually moved forward, pushing the students back every few minutes, until the officers rushed the crowd and made more arrests.

Before long most demonstrators had retreated, police held the otherwise empty plaza strewn with trash, and a few onlookers remained at the periphery. (Reuters)