WASHINGTON (Reuters) - In a rare move, the U.S. government announced late Sunday that only emergency workers should report to work Monday in a large area surrounding the International Monetary Fund and World Bank, where thousands of protesters were expected again to target world financial meetings.

A spokeswoman for the Office of Personnel Management (OPM) said nonessential federal workers at the State, Treasury, Commerce and Interior departments, and other key agencies in the area would have a rare -- and unexpected -- day off, at the recommendation of federal and local law enforcement officials.

Washington Police Chief Charles Ramsey predicted heavy traffic during the morning rush hour Monday as protesters tried to block financial leaders gathering for the sixth and final day of meetings with the International Monetary Fund and World Bank.

Anti-globalization protesters, who claim the institutions' policies hurt the poor, tried to block financial leaders from meeting Sunday, but they were beaten back by police wielding batons and squirting pepper spray.

Monday's partial shutdown, rarely used except for weather-related emergencies, affects a large area bounded by 12th Street N.W., 23rd Street N.W., Constitution Avenue and K Street N.W., OPM said.

Outside that area, government agencies would be operating, but workers could take unscheduled leave at their own discretion, OPM said.

"This is obviously a decision that we don't take lightly. It's very unusual and very rare," a spokesman for the agency said.

The last time federal workers stayed home, aside from some snow days this past winter, was during the NATO summit in Washington last year when officials were also concerned about security.

The OPM spokesman said the security situation surrounding the World Bank and IMF talks would be reviewed again Monday and a decision would then be made on whether to extend the partial shutdown.

"We always have to balance the important things that we in government do with the safety of federal workers," the spokesman said.

Police arrested 20 protesters Sunday, bringing to 657 the number detained during in the last two days of demonstrations and dwarfing the 525 arrested during a week of similar protests in Seattle last December.