Suspected triad members are among seven people arrested over scuffles that broke out between pro- and anti-government camps outside a Tin Shui Wai event attended by Chief Executive Leung Chun-ying on Sunday.
A political academic warned that the alleged triad link to the pro-government camp might inflict further damage on the government.
In the latest arrests, two men, aged 22 and 24, were held on Monday night for common assault, after four people were caught on camera beating a member of the pan-democratic League of Social Democrats outside Leung's meet-the-people session. A 19-year-old man was detained earlier in the day.
The trio were suspected to belong to triads active in Tuen Mun and Yuen Long, a police source close to the matter said. Officers were looking for the fourth man, aged between 20 and 25, and any co-conspirators behind the assault.
Four others, including league vice-chairman Avery Ng Man-yuen, were arrested on Sunday for common assault and disturbing public order.
The league condemned the police for failing to take immediate action on Sunday or "to enforce the law in a fair way".
"I was grabbed by more than 10 officers right after I threw a folding chair in front of Leung's car without hurting anybody," Ng said. "At the same time my colleague was beaten up for more than 30 seconds, but no police nearby took immediate action."
Now that the triads, pro-establishment camp and police had joined forces to oppose dissidents, "Leung has officially declared war on pan-democrats".
Leung will hold another meet-the-people session in Kwun Tong this Sunday. Ng suggested pan-democrats adopt a new tactic - silent opposition.
"We could all dress in white and simply stand still in the protest outside the meeting, to express our ultimate opposition to the pro-government alliance's violent acts," he said.
The police rejected the criticism, saying they always handled cases in an impartial manner.
Political scientist Ma Ngok, of Chinese University, criticised the possible triad link. "Leung's administration is acting like other dictatorial regimes around the world that hire triad members to attack dissidents."
He believed Sunday's clash was part of the pro-government camp's plan. "People might have reservations about the Occupy Central movement for democracy after witnessing the scuffles."