HONG KONG — The riot police, dressed in green fatigues, advanced down a central city street. Some of the officers wore black masks, while others held shields or shotguns loaded with rubber bullets and tear-gas grenades.

As a few dozen protesters jeered, police fired back with their own insults.

“Remember, you are not a person, you are not even worthy as an animal!” an officer shouted. “You are not a person, you’re a cockroach!” Soon, police unleashed tear gas at the crowd from close range — one of almost 400 canisters fired on Saturday.

Derogatory language — with protesters terming officers dogs and gangsters, and police calling demonstrators subhuman and cockroaches — has become a hallmark of Hong Kong’s protests as clashes have escalated.

Several people were injured in a knife attack at a Hong Kong mall Nov. 3, including a local politician whose ear was partly bitten off. (Reuters)

Police put up a massive show of force this past weekend— even against peaceful protesters — as authorities grow more determined to end the months-long unrest. Between Friday and Sunday, more than 300 people were arrested, according to police. On Sunday, a man armed with a knife attacked pro-democracy demonstrators and left three in critical condition, including one who had part of his ear bitten off.

In this deteriorating climate, the dehumanizing language employed by police is especially troubling, experts say, as it can be a precursor to more indiscriminate violence toward civilians.

The use of “cockroach” — a slur used by the Nazis against Jews and by Hutus against Tutsis in the Rwandan genocide — often provides the rationale for harsh and cruel treatment against a group of people and serves to further polarize society, these experts say, especially when used by state actors.

“There is the notion of an infestation, that there’s something here you need to snuff out before it takes over,” said Allison Skinner, an assistant professor of behavioral science and social psychology at the University of Georgia. Cockroaches, she added, are “very, very low. It emboldens anyone using or hearing this language to feel like it is almost their obligation to respond to them with force and violence.”

In July, the Junior Police Officers’ Association, a union that represents many front-line officers, started referring to protesters in its official statements as cockroaches. In an Aug. 4 statement, the association wrote: “Even though the rioters don’t like being called ‘cockroaches,’ how you’re acting is indeed like cockroaches. Cockroaches, please stop.”

When police cleared a group of protesters earlier in the summer, they referred to them as cockroaches. They can be heard shouting “Cockroaches, run away!” while shooting tear gas and projectiles at fleeing protesters.

“This sort of derogatory language can lead to a deeper kind of genuine dehumanization,” said David Livingstone Smith, a professor of philosophy at the University of New England who has written books on dehumanization and violence. “You call people cockroaches a lot, you start thinking that they are subhuman.”

Such language, he added, must be viewed “either as a symptom of an increasingly dangerous situation, or a promoter of one, or both.” He noted that such terms were used in Nazi Germany and in Rwanda long before the atrocities there reached their zenith.

Asked in August about the matter, Kelvin Kong, a senior superintendent with the police public relations branch, told reporters that such language from front-line officers was “not ideal” but added that all parties “are under a lot of stress.”

That month, an assistant commander of a police district, in an email obtained by the independent Hong Kong Free Press,warned that officers should not refer to protesters as “cockroaches” to avoid “playing into their hands.”

In an emailed response to questions from The Washington Post, the police public relations branch said the department is “aware of such a term used by certain officers and are of the view that the language is not appropriate.”

“Officers have been reminded to always present themselves professionally and to use respectful language to address members of the public,” the response added. “However, officers’ interaction with members of the public is based upon mutual respect. It is understandable that given the vulgar verbal abuse from certain radicals targeting the police, officers may have ups and downs in their emotions.”

Trust in Hong Kong’s police force, once deemed “Asia’s Finest,” has collapsed in recent months. In polling conducted last month by the Center for Communication and Public Opinion Survey at the Chinese University of Hong Kong, 51.5 percent of respondents said they had zero trust in the police force, compared with 6.5 percent in May and June.

On Sunday, a 24-year-old who gave only one name, Lau, for fear of repercussions, attended an unsanctioned rally in Victoria Park with her aunt and friend. They left before police stormed in. The three were stopped by officers as they were exiting, she said in an interview, and told to walk in another direction.

The trio tried to explain to the officers that they lived only one block away, in the direction they were headed, but officers searched their bags, briefly misidentifying a showerhead that Lau’s aunt had purchased earlier in the day as a box of masks.

As the search was finishing, Lau said, an older male officer told them: “Okay, let me give you some advice. Go back up [to your home], stop being cockroaches, and just become humans again.”

Lau was stunned. “I’ve heard them say it on TV, and I’ve heard people describe how they were called cockroaches, but I guess it was like getting the wind knocked out of you,” she said. “I felt cold because he said it so matter-of-factly.”

As concerns have grown that the police are operating with impunity, even moderate politicians and pro-Beijing lawmakers have urged the city’s leader, Carrie Lam, to open a fully independent investigation into the force, which is one of the protesters’ demands. She has refused.

The language from the police underscores the lack of accountability under Hong Kong’s undemocratic system, said William Donohue, a professor of communication at Michigan State University who studied the function of dehumanizing language in the Rwandan genocide.

“If the state actor comes from a democratically elected system, you typically don’t get this kind of language [as] there is the assumption that the government is accountable to the people,” Donohue said.

“But when you have a government that is not elected, when it rules by other means, they feel less restricted, they are not accountable to anybody.”

Tiffany Liang and Anna Kam contributed to this report.