New York (AFP) - As a storm broke over New York, riot police mounted a tall ladder on a reinforced truck to penetrate a building at Columbia University and clear out barricaded pro-Palestinian protesters.

Scores of armed, helmet-clad NYPD officers arrived in the north-west of Manhattan on Tuesday evening, accompanied by buses and trucks, as they set up barriers around the grounds of the prestigious institution.

Columbia, which counts 37,000 students and is typically open to the public, was entirely surrounded.

Called into action after a request for help from university president Minouche Shafik, police began arresting protesters outside the main gates of the campus.

The Ivy League college -- home of the student movement against the Vietnam War in the late 1960s -- has for two weeks been the epicenter of anger over Israel's war on Gaza.

'Shame on you'

"Free, free Palestine!" and "Shame on you," protesters cried as law enforcement officials. 

Dozens were handcuffed and ushered into buses and police vans, according to an AFPTV journalist at the scene.

Then, accompanied by a huge truck equipped with a large ladder, police entered the campus and approached Hamilton Hall, a university building occupied by protesters since Tuesday. 

Officers wearing riot gear then climbed the ladder to reach a window and entered, one by one. Police arrested dozens of people, according to CNN.

They were loaded into police buses and vans. 

Outside campus, an AFP reporter saw people handcuffed using zip ties sitting inside one van that carried about a dozen people.

Pro-Palestinian group Columbia University Apartheid Divest said police was "violently invading" the campus, accusing it of "brutally attacking" protesters.

'Escalation'

On campus, several dozen police surrounded all perimeters of the school and pushed students into dorms or out of the campus altogether. 

Hundreds of students who left their dorms to come onto the streets were stranded and unable to return to their homes. 

A student studying at Columbia's School of International and Public Affairs said police also blocked her from going to her dorm, which was not on the main Columbia campus where protesters were located.

Columbia's president, who had last Friday pledged not to call in the police to dismantle a tent camp set up by protesters on campus since mid-April, changed tack following the storming of the Hamilton Hall. 

Denouncing an escalation in the situation, the university threatened "students occupying the building" with "expulsion", accusing them of vandalizing, breaking and blocking access to the building. 

Over the past two weeks, American student anger has spread from the major universities on the East Coast to those in California, via the South and Central regions, recalling the anti-Vietnam War protests of the late 1960s. 

Columbia is also at the heart of the movement against Israel's war against Hamas in Gaza. 

A tent "village" housing up to 200 people was set up on a lawn in mid-April, foreshadowing an intervention.

At around 0500 GMT on Wednesday, an AFP journalist saw the encampment was cleared with all tents taken down by university staff and leftover items thrown away in giant black rubbish bags.

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