Leaders and supporters of the Occupy Central with Love and Peace movement, many wearing plastic capes and goggles to fend off any police pepper spray attack, had urged more of the public to join the protest to pressure Beijing to allow free elections in the former British colony.
Hundreds of police, donning helmets, were cordoning off streets and walkways in Admiralty district after tension flared amid the arrests of several lawmakers and the seizure of audio equipment.
The move raised fears that the police were preparing an extensive crackdown before Hong Kong opens for business on Monday, Reuters reported.
"Anyone attempting to forcefully enter the place might be arrested," the South China Morning Post quoted police public relations branch Superintendent Kong Man-keung as saying.
Kong did not say there would be a full clearance, stating only that "we will take the appropriate action based on assessment of the circumstances".
Publishing tycoon Jimmy Lai, a key backer of the democratic movement, said he wanted as big a crowd as possible, after a week of student demonstrations, to thwart any crackdown.