Groups of men, many wearing surgical masks, descended on the front lines of the rally at Admiralty near the city's central business district, triggering clashes with protesters after police had moved in to take down some barriers.
Demonstrators, who have come under attack from organised crime gangs known as triads at another flashpoint demonstration site in Mongkok, shouted: "Weapons! Weapons!" and "Arrest the triads" as police struggled to impose order.
Despite repeated orders to disperse, rallies now into their third week have taken on an air of permanence, with tents, portable showers and lecture venues springing up in recent days.
"This is one of the tactics used by the communists in mainland China from time to time. They use triads or pro-government mobs to try to attack you so the government will not have to assume responsibility," Democratic Party lawmaker Albert Ho told AFP.
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"It seems that the police have duly removed some of the barricades to make way for the suspected triads to get through to the peaceful protesters," Civic Party lawmaker Claudia Mo told AFP.
Anti-Occupy protesters mainly dissipated as the day wore on, while pro-democracy demonstrators set about reinforcing their positions in case the police return for a second dawn raid tomorrow.
In Admiralty activists laid down cement foundations and built up bamboo pole barricades blocking both lanes of a highway, using everything from steel chains to plastic ties and sticky tape to strengthen the structures, even enlisting sympathetic construction workers for help with their building work.
Protesters also filled plastic barricades with water, making them difficult to move, after police warned Monday they would move to clear more "obstacles" from the protest sites.