Police in Washington, DC have arrested 31 anti-Wall Street protesters after authorities moved in to remove a wooden structure the protesters erected without a permit.

Protesters, who have been camped at McPherson Square for weeks, began erecting the frame for a two-story wooden building Saturday. But police informed them Sunday morning they needed a permit and gave them an hour to dismantle the structure.

When that did not happen, police moved in, arresting protesters who straddled the top of the building framework.

Members of the Occupy Wall Street movement, the protesters are demonstrating against economic inequality and what the see as corporate greed. They say the building was intended to be shelter in the coming winter weather.

Occupy Wall Street camps in cities around the country have been closed by authorities in recent weeks.  In the northwestern city of Portland, Oregon, police shut down a downtown-area park Saturday night, arresting anti-Wall Street protesters who refused to leave.

The encampment at Zuccotti Park in New York, where the movement began, was shut down in mid-November.

Until now, Washington police had avoided the clashes that have taken place in other cities.