Outside the Aberdeen Centre mall in Richmond, near Vancouver, supporters of the Hong Kong protest movement were marking China’s National Day on Tuesday by building a Lennon Wall – and counterprotesting opponents were tearing it down.

Taunts and challenges turned into shoving. Then there were cheers and applause when uniformed police appeared on the scene and led away two of the counterprotesters, to shouts of “go back to China” and “stand with Hong Kong”, video of Tuesday’s incident shows.

But unseen by the 50 or so protesters and about a dozen opponents were undercover officers from the Royal Canadian Mounted Police (RCMP) who were already monitoring the scene for potential clashes, a police spokesman said.

The plain-clothes operation points to the rising tensions in Richmond, the Vancouver satellite that is the most Chinese city in the world outside Asia.

Fifty-four per cent of the city of roughly 200,000 is ethnically Chinese. Richmond was once a stronghold of Hong Kong immigrants, but mainland Chinese immigrants now outnumber them two to one.

RCMP officers with the Richmond detachment “were aware of the gathering at Aberdeen Station and had planned for the event”, Corporal Dennis Hwang said on Wednesday.

“We are acutely aware of current world events, especially those that correspond to the October 1st date. We do extensive planning for such events and balance the needs of peaceful protest with public safety. We had officers stationed nearby and in plain clothes to monitor the situation.”

Hwang said the goal of an undercover operation was “always public safety”.

“That’s it. There’s nothing more than that. It’s nothing nefarious.”

Asked what moment caused the uniformed officers to intervene, Hwang said: “I’m not going to specify which particular matter … but it’s our own undercover officers that may have said ‘hey, you need some uniformed officers to come in and assess that situation’.”

There were no arrests.

The Lennon Wall event was organised by local high school students, supporters said. Usually made of Post-it notes carrying messages of support, Lennon Walls have been prominent features of the protest movement in Hong Kong and beyond – and frequent targets of opponents.

Two people who attended Tuesday’s event and supported the Lennon Wall said they were unaware of any police presence before the uniformed officers swooped in.

Neither was aware of anyone telephoning the police. “I can only say that police came after the situation got tense,” said one of the supporters, who declined to be identified.

Mandarin-speaking opponents of the display, some wearing designer-labelled clothing, hurled money at supporters of the display before the clashes broke out, both witnesses said. They challenged the crowd to fight, video shows.

“One individual threw toonies (C$2 coins) at us as an insult to tell us to take the bus home,” said the first protest supporter.

The second said that coins were also thrown at a middle-aged woman who supported the schoolchildren. “One of the counterprotesters threw a bunch of coins … I think it was a class thing,” said the 34-year-old Hong Kong immigrant, who gave his name as Brian.

“All these young counterprotesters are dressed in high-end fashion brands, you know, Balenciaga, expensive clothes, so I guess they were throwing money at us people just wearing Uniqlo.”

Class differences have previously been a feature of clashes over the Hong Kong protest movement in Canada. Demonstrations in Vancouver and Toronto in mid-August were buzzed by drivers of Ferraris and other sports cars draped in Chinese flags. Vancouver has attracted thousands of Chinese millionaires in recent years, under wealth-migration schemes.

The violence outside Aberdeen Centre did not escalate beyond the shoving seen in videos of the incident, the witnesses said. A young supporter of the wall is seen trying vainly to defend the display. He and one counterprotester exchange hand slaps; when another advances on him, the young supporter gives him a shove.

When a uniformed officer arrives and leads away two counterprotesters, he has to wave back the pair’s crowd of opponents who chant Hong Kong protest slogans.

Both attendees interviewed by the South China Morning Post said they were grateful for the police action, in contrast to the anger directed at Hong Kong police by protesters there. However, they said police spoke to the Lennon Wall organisers and told them to take down the tattered remnants of the wall.

A few hours after the clashes, someone had started rebuilding the Lennon Wall again, social media photos show.

Nevertheless, the unnamed attendee said he was thankful for the police intervention “because we felt threatened”.

“I got the impression that the police only want to de-escalate the situation, which is what they are taught to do,” he said.

But the situation felt “ridiculous” in Canada, he said. “I do feel frightened, not for myself but for our younger students. This level of intimidation would have been horrifying if there were fewer people [supporting the wall],” he said.

Hwang, of the RCMP, said he could not characterise the interactions between the police and the people on either side with whom they spoke, or whether they were questioned or warned.

He said that uniformed officers relied strongly on undercover officers’ assessments. “They are the most trusted eyes on the ground, they are the ones who can make a judgment call better than [anyone]. They know what they’re dealing with. If it looks like a situation that can spiral or escalate, so without giving their position away, they can call other officers.”

He said there was no political aspect to operational decisions. “Our response could be the same for job action [in a labour dispute],” he said.

Without talking specifically about tensions surrounding Hong Kong, Hwang added: “We are always around. You may not notice us. But we have to keep an eye on it.”