Police in Los Angeles began removing barricades early Thursday morning at a pro-Palestinian demonstrators' encampment on the University of California, Los Angeles (UCLA) campus. The law enforcement effort came after officers spent hours threatening arrests over loudspeakers if people did not disperse.

Police detained a handful of people on campus, tying their wrists with zip ties. The police action occurred a night after the UCLA administration and campus police waited hours to stop the counter-protesters' attack. The delay drew condemnation from Muslim students and California Gov. Gavin Newsom.

As police helicopters hovered overhead, the sound of flash-bangs, which produce a bright light and a loud noise to disorient and stun people, pierced the air. Protesters chanted "where were you last night?" as the officers approached.

Demonstrators had rebuilt the makeshift barriers around their tents on Wednesday afternoon while state and campus police watched.

Shortly before 2 a.m. local time, police briefly made their way into the perimeter of the encampment only to retreat after being outnumbered by scores of protesters who yelled, "Shame on you!" Some in the crowd tossed water bottles and other objects as dozens of officers ran back. Later, the crowd chanted, "We're not leaving. You don't scare us."

Armed with batons and in full riot gear, California Highway Patrol officers returned about an hour later and stood within feet of scores of protesters, who threw objects and yelled. The huge numbers of police began arriving late in the afternoon Wednesday and issued the dispersal order.

Police response criticized

Tent encampments of protesters calling on universities to stop doing business with Israel or companies they say support the war in Gaza have spread across campuses in the U.S., with several also arising in Canada. The ensuing police crackdowns echoed actions decades ago against a much larger protest movement opposing the Vietnam War.

The tense standoff at UCLA came one night after violence instigated by pro-Israel counter-protesters erupted in the same Royce Quad area. The law enforcement presence and continued warnings stood in contrast to the scene that unfolded the night before, when counter-demonstrators attacked the pro-Palestinian encampment, throwing traffic cones, releasing pepper spray and tearing down barriers.

WATCH | McGill protesters want more than just talks on Israeli divestment: 

What the McGill University protesters want

For days, pro-Palestinian protesters have been camped out on the McGill University campus in Montreal. CBC’s Jayme Poisson is there and breaks down the group’s demands and how it’s responding to crackdowns on similar encampments at U.S. universities.

At least 15 protesters suffered injuries, and the tepid response by authorities — no arrests were made — drew criticism from political leaders as well as Muslim students and advocacy groups.

"The community needs to feel the police are protecting them, not enabling others to harm them," Rebecca Husaini, chief of staff for the Muslim Public Affairs Council, said in a news conference on the Los Angeles campus Wednesday.

Speakers at the news conference disputed the university's account that 15 people were injured and one hospitalized, saying the number of people taken to the hospital was higher.

UCLA Chancellor Gene Block said in a statement that "a group of instigators" perpetrated the previous night's attack, but he did not provide details about the crowd or why the administration and school police did not act sooner.

"However one feels about the encampment, this attack on our students, faculty and community members was utterly unacceptable," he said. "It has shaken our campus to its core."

Block promised a review of the night's events. The head of the University of California system, Michael Drake, ordered an "independent review of the university's planning, its actions and the response by law enforcement."

Hundreds of arrests in U.S.

The chaotic scenes at UCLA came after New York police burst into a building occupied by anti-war protesters at Columbia University on Tuesday night, breaking up a demonstration that had paralyzed the school.

The nationwide campus demonstrations began at Columbia on April 17 to protest the humanitarian situation Gaza, resulting from the Israeli military response to a deadly attack on Oct. 7 launched by Hamas and other militants.

About 1,200 people were killed on Oct. 7, including several Canadians, while some 250 people were taken hostage, according to Israeli government tallies. Since then, more than 34,000 Palestinians have been killed in the Gaza Strip, according to the Health Ministry there.

An Associated Press tally counted at least 38 times since April 18 where arrests were made at campus protests across the U.S. More than 1,600 people have been arrested at 30 schools.

In Madison, Wisc., a scrum broke out early Wednesday after police with shields removed all but one tent and shoved protesters. Four officers were injured, including a state trooper who was hit in the head with a skateboard, authorities said. Four were charged with battering law enforcement.

Israel and its supporters have branded the university protests antisemitic, while Israel's critics say it uses those allegations to silence opposition.

WATCH l No signs of campus protests abating:

U.S. campuses see clashes, arrests as Canadian encampments grow

After arrests at New York's Columbia University, Los Angeles police appear to be preparing to move in on protesters at UCLA. The number of encampments in solidarity with the Palestinian cause continue to grow as students camped at Montreal's McGill University say they're not going anywhere.

Although some protesters have been caught on camera making antisemitic remarks or violent threats, organizers of the protests, some of whom are Jewish, say it is a peaceful movement aimed at defending Palestinian rights and protesting the war.

At Brown University in Rhode Island, administrators agreed to consider a vote to divest from Israel in October —apparently the first U.S. college to agree to such a demand.