WASHINGTON - Police arrested 31 protesters at an anti-Wall Street encampment in Washington on Sunday and tore down a wooden shelter that activists illegally erected overnight, prompting a nine-hour confrontation.

Officers on foot and horseback converged on McPherson Square, near the White House and where demonstrators have been living since late September, leading several activists to climb onto the structure and refuse to come down.

After making dozens of arrests on Sunday near the roofless hut some 8 meters tall, US Park Police deployed an armored car and motorized lift to try to remove six hardy demonstrators who had clung on limpet-like to the shelter.

Ladders were then used to ease down the last protester who for more than half an hour held off five officers who had tried to attach a rope to his body to lift him off the structure. But he was eventually prised off its beams, and a fork-lift truck later moved in and flattened it.

A crowd of around 400 people had gathered to watch the unfolding spectacle, and many of them shouted "shame on you" at police as arrests were made at the site while also chanting "liberty and justice will prevail".

Despite moves to evict "Occupy" protesters from similar camps in New York and other cities, authorities in the US capital have generally refrained from taking action against the village of tents and tarps that has sprung up here.

But the erection of a wooden building, against regulations, prompted officers to swarm into the square around 11:30 am, and the standoff ensued.

Sergeant David Schlosser, a park police spokesman, said that 31 people were arrested and charged - 15 of them for crossing a police line and 16 for defying police orders.

A police officer at the scene said earlier that the people who had climbed on to the structure's wooden beams were given three warnings that they were subject to arrest.

The park police were backed by District of Columbia officers who surrounded the square, the focal point of Occupy DC, and sealed off surrounding streets.

The anti-Wall Street movement, born in New York in mid-September, is trying to redefine itself as permission to camp in public spaces is being rapidly curtailed.

The loosely organized, left-leaning Occupy Wall Street protesters insist they are exercising their freedom of speech in the run-up to November 2012 national elections.

Following New York's Zuccotti Park eviction in November, police cleared out protesters this week in Los Angeles and Philadelphia.

There have also been evictions in Dallas, New Orleans, Oakland and Portland, and some clashes with police.

Agence France Presse