Several scuffles had already broken out between protesters and a group of about 10 plain-clothes officers, who were quickly identified as police after they entered the mall.

In one instance, pepper spray was used as a mob tried to charge at police after they briefly detained a protester said to have threatened one of the officers.

A youngster who was pepper-sprayed said riot police rushed into the mall, urging everybody to leave. But he said they had no way to move away and riot police sprayed them anyway.

Meanwhile, masked radicals struck after a sit-in at Yoho mall to mark the five-month anniversary of the Yuen Long MTR station attack on protesters and passengers, vandalising a Genki Sushi restaurant before police arrived. The outlet was targeted in the mall, next to the station, after about 100 people had gathered to commemorate the July 21 attack, which was carried out by a group of white-clad men and left dozens injured.

Dozens of riot police arrived to disperse protesters and onlookers in the mall and were heckled. About 20 black-clad protesters then headed to Castle Peak Road at about 4.50pm and smashed traffic lights. An onlooker said protesters hit a man on Tai Tong Road after he used foul language against them. Blood was spilled and the man was taken away by ambulance.

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In the mall, police searched at least one person. Riot police returned to the mall at around 9pm, formed a cordon, and arrested several people for criminal damage. An outlet of Heytea, a mainland China-based beverage firm, was targeted.

Radicals frequently target restaurants operated by Maxim’s Group, including Genki, after its founder’s daughter called protesters “rioters”, and said they did not represent Hong Kong. They also focus on shops and banks with links to the mainland.

Earlier, those at the sit-in aired grievances against police, who they said failed to respond quickly enough to the July 21 attack, which resulted in 45 people needing hospital treatment.

“Dismiss the police force, no more delays,” the crowd chanted.

The MTR Corporation shut down Yuen Long station at 1pm on Saturday.

In Harbour City, protesters went to shops considered “red” or “blue”– those with links to mainland China or supporting the government – and urged people not to buy from them. They also visited food outlets they regarded as pro-government and told people to stop eating there.

“If you know [this restaurant] supports police brutality, I hope you will stop eating here,” a male protester shouted as he and others went into a restaurant.

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Another tactic they adopted was to ask mainland tourists not to buy any foreign products, shouting in Mandarin sarcastically: “Love China. Buy national goods.”

Numerous shops closed their gates as protesters passed, and shoppers thinned out in the originally crowded mall, heading for the exits.

Cheng, 65, a retired accountant, joined the protest. She said she took part in almost all of the protests in recent months, although she did not support all of the tactics used.

“It is not good for them to enter ‘blue shops’ and scare consumers away as it should be up to people to choose out of their conscience,” she said. “But I understand their anger.”

At about 9pm, Tsang Chung-bun, police’s assistant commander of Yau Tsim district, wrapped up the Harbour City operation, saying one man had been arrested for allegedly assaulting an officer. He also denied an officer had pulled out his gun, but said police would be in an alert position by pressing a hand on it.

Two young protesters, both 14, said the “shopping” had achieved the aim of forcing stores to shut.

“We want to crack down on the local economy, to exert pressure on the Hong Kong government and Beijing,” Form Three pupil Alex said. “The goal was actually achieved with the help of riot police, as more shops closed after they rushed in.”

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His friend Christine said she supported such “peaceful shopping” but was worried about previous vandalism in the mall.

Cheng said those who resorted to violence and vandalism have had to pay the price, being among those arrested.

“The government should be the one that is scared, as protesters have escalated their violence and even live ammunition was found,” she said. “The Hong Kong government and Beijing should take the lead to cool down society, or else there is no room to bargain.”

Police were on guard in nearby areas. Around 80 riot police officers were stationed on Canton Road, a busy shopping street on which there are entrances to the mall.

Protesters quickly identified a group of plain-clothes police who had followed them into the mall.

The officer in charge showed his warrant card and urged them to stay peaceful after protesters surrounded them. At one point, two protesters threw dog food on the ground, saying: “Sirs, time to have a meal!”

The pair were detained on suspicion of behaving in a disorderly manner in a public space. They were ordered to pick up the food and later allowed to leave.

Across the harbour in Central, hundreds of middle-aged and elderly people gathered in Edinburgh Place for a rally. They called on the government and public bodies not to appoint defeated district council election candidates to committees.

On Saturday night, a few people staged a peaceful sit-in in Heng Fa Chuen MTR station on Hong Kong Island to mark the July 21 attack.

Reporting by Sum Lok-kei, Chan Ho-him, Kimmy Chung, Simone McCarthy, Lilian Cheng and Elizabeth Cheung