Four NATO protesters were arrested Tuesday at a rally outside a Chicago immigration court ahead of a weekend summit that will bring dozens of world leaders to the Windy City, police said.

Television crews captured protesters confronting police on bicycles positioned outside the building and chanting "this is what democracy looks like."

Thousands of protesters are expected to descend upon Chicago as the leaders of 50 countries gather for a NATO summit on Sunday and Monday.

On Monday, eight protesters were charged with trespassing after they refused to leave the Chicago office tower which houses President Barack Obama's campaign headquarters.

Fears that the protests could turn violent have put the city on edge, with some downtown businesses even telling office workers to ditch their suits and ties and dress down to avoid being hassled or targeted on the streets.

Police and protest organizers have vowed that there will be no repeat of the trouble that erupted at events such as G20 summits in London and Toronto or the riots which scarred Chicago during the 1968 Democratic National Convention.

The decision to move the G8 summit -- set for Friday and Saturday -- from Chicago to Camp David is expected to lessen the intensity of demonstrations in Obama's adoptive home town.