Protesters from the Washington offshoot of Occupy Wall Street vowed to return to their camp in the US capital, which was all but dismantled by park police during a weekend clampdown.

A dawn raid and day-long enforcement operation on Saturday stopped activists from the anti-capitalist movement camping out at McPherson Square, near the White House, overnight and the site was reduced to around 30 tents on Sunday.

Sanitation crews in white overalls, backed with forklifts and garbage trucks, swept up the belongings left over from the months-long protest, while a police truck with an infrared scanner was used to ensure no-one sleeps there.

The enforcement operation led some of the activists to spend Saturday night at nearby Freedom Plaza, another protest site, although it was subject to the same no-camping rules.

Despite the clampdown, protester Sam Mellot said McPherson Square would be "occupied" again.

"Any time you see an occupation getting torn down they come back stronger. We'll come back -- absolutely we'll come back stronger," he said Sunday.

Occupy DC -- in the heart of the K Street lobbying district -- took root on October 1, swelling over time to around 100 tents that included a library, a cafeteria, and a medical clinic.

The site sprung up a few weeks after Occupy Wall Street protesters took up tented residence at New York's Zuccotti Park, near the Big Apple's financial district.

The issues protesters raised on income equality and big-business influence on politics reverberated across the nation and took on extra resonance ahead of the November general election, but the movement has since stalled.

US President Barack Obama has described battling income equality and maintaining the American dream as "the defining issue of our time," as protesters have sought to push Democrats to take action.

Leading candidates for the Republican nomination have been met at rallies in the early voting states by "Occupy" protesters calling on them to address the issue, but Washington was the movement's last tent encampment.

On Sunday, authorities in the US capital continued what they called "nuisance abatement activities" at Freedom Plaza, making sure protesters complied with regulations against camping -- defined as use of park land for "sleeping activities."

Tents were allowed at the site only if they were empty of personal belongings.

Protesters denounced what they called "excessively forceful eviction," and gathered to discuss the movement's direction after the clampdown.

"This occupation is far from over. They can't evict an idea whose time has come," said a statement posted on the website

In total 11 people were arrested Saturday, four for assaulting police officers, including one taken into custody for hitting and injuring an officer with a brick, and seven for disobeying orders to clear out, or for crossing police lines, a park police spokesman told AFP.

A 12th protester was arrested on Sunday.

Occupy DC was informed on January 27 that the no-camping regulations would soon be enforced, after Washington authorities deemed the situation at the park unsuitable on health and safety grounds, citing the presence of rats.

On Sunday morning the clean-up crew jumped at the sight of a scurrying rodent -- shouting "rat!" as it scrambled under their feet.

Asked how many rats they'd found since clean-up began, one of the crew told AFP "hundreds."