"People are saying the police are switching back to being licensed triads," said a protester who gave his name as Wong.
Their reputation took a further battering when several officers were caught on video beating and kicking a handcuffed protester as others kept lookout.
The triad accusations hark back more than four decades to an era of violent unrest fomented by the Chinese Communist Party in the grips of Mao Zedong's Cultural Revolution and to a colonial-led force that was mired in graft and brutality.
The then Royal Hong Kong Police worked hard to clear that stain and earn a place among the world's most trustworthy forces.
According to the World Justice Project's 2013-14 rule of law index, only police forces in Japan, Singapore and Denmark ranked higher in providing order and security. Hong Kong police were also fourth - behind Finland, New Zealand and Qatar - in a World Economic Forum 2013-14 global competitiveness report that gauged the reliability of police services.
"Great deal of restraint"
Criticism of Hong Kong police was misplaced given the "quasi-military" tactics adopted by some countries, including the United States, said Allan Jiao, professor in the department of Law and Justice Studies at Rowan University, New Jersey.
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"Hong Kong police have exhibited a great deal of restraint in the face of the protests and their performance compares favourably internationally," he said. "Occupying major street areas for a prolonged period of time would not be tolerated in the U.S. or U.K."
In Mong Kok, a crowded area where market stalls and massage parlours are neighbours to jewellery shops and noodle restaurants, frustrations boiled over when protesters charged police one night, reoccupying two busy streets.
The next evening, police reinforcements weighed into the pro-democracy crowd with shields, batons and boots. Unusually, police hurled back insults that would typically have gone unanswered.
"We have been trained to be patient and self-disciplined. But sometimes tolerance has a limit, especially when people are continuously using foul language," said one constable who declined to be identified because he was not authorised to speak to reporters.
Morale was at an all-time low among junior police, the South China Morning Post cited Junior Police Officers' Association chairman Joe Chan as saying. In-house police counsellors were visiting frontline officers to provide support, the Police Public Relations Branch said.
"Officers have to work for prolonged hours to handle large number of protesters and to face provocation, insult, attack and groundless allegations against them by radical trouble-makers," it said in emailed answers to questions.
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If there is a precedent for what the force is facing, it is the 1967 communist-led riots, said Kam Wong, an associate professor of criminal justice at Xavier University in Cincinnati and a former Hong Kong police inspector.
What started out as labour disputes at a plastic-flower factory and elsewhere turned into a fully fledged attack on British rule. When an outspoken radio host was doused with petrol, set ablaze and killed by assailants believed to be aligned with the rioters, public opinion turned and the police moved in, Wong said.
The people at the time finally felt that enough was enough, he said. "The police needed to take action."
There has been no violence on the scale of 1967 over the last few weeks. But growing frustration over chronic congestion and lost business as the protests drag on may yet hand the police a mandate to clear the demonstrators off the streets.
"The use of batons at close quarters, the use of tear gas - all these things have undone the good work re-establishing public trust in the Hong Kong police after the 1960s," said Carol Jones, a professor of socio-legal studies and criminology at the University of Wolverhampton.
"I think a whole generation now will no longer trust the police."