Australians travelling to Hong Kong have been warned to exercise a “high degree of caution” as China said Hong Kong was facing its worst crisis since the former British colony was handed back in 1997.

The Australian Department of Foreign Affairs and Trade (Dfat) said: “There is a risk of violent confrontation between protesters and police, or criminally linked individuals, particularly at unauthorised protests.” Ireland, the UK, and Japan have all issued Hong Kong travel warnings since July.

Zhang Xiaoming, one of the most senior Chinese officials overseeing Hong Kong affairs, spoke on the issue at a meeting in the southern Chinese city of Shenzhen to discuss the crisis.

“The central government is highly concerned about Hong Kong’s situation, and trying to study, make decisions and arrangements from a strategic and across-the-board level,” Zhang said in opening remarks. “Hong Kong is facing the most serious situation since its return to China, therefore today’s seminar is very important.”

On Tuesday night Hong Kong police clashed with about 100 protesters and residents of the working-class district of Shamshuipo after a student union leader was detained for carrying a laser pointer.

A Hong Kong riot police officer carries supplies to the front line during the clash with protesters. Photograph: Miguel Candela/SOPA Images/REX/Shutterstock

The police held Keith Fong on the charge of possessing an offensive weapon, according to the Facebook page of the Hong Kong Baptist University Student Union, which accused the police of fabricating the charge in order to arrest people arbitrarily and called for his immediate release.

On Wednesday a message was circulating on the messaging app Telegram saying Fong’s arrest “precisely shows the power of the extradition bill ... they can arrest you on a whim.”

Dfat said the protests were expected to continue and had become more unpredictable and strongly recommended staying away from large public gatherings, adding that the risk was greater at night and on weekends.

As of Wednesday, the US state department and the Canadian government still had their alert to “normal precautions,” for the city.

Hong Kong is in its ninth week of consecutive mass protests, and police on Tuesday said they fired 800 canisters of teargas during protests on Monday as they looked to clear demonstrators from at least seven districts across Hong Kong.

On Tuesday, shops were closed in North Point from mid-afternoon after rumours circulated that busloads of people were coming from Fujian province in mainland China. Other reports suggested protesters would be going after the men in white who attacked them the previous night.

The protests began in opposition to a now-suspended extradition law, which would have allowed suspects to be tried in mainland Chinese courts. They have now broadened into a backlash against the government of the Asian financial hub, fuelled by many residents’ fears of eroding freedoms under the tightening control of China’s Communist party leaders in Beijing.

More than 5 million people visited Hong Kong in June this year, of which roughly 80% were from mainland China.

June figures from the Hong Kong Tourism Board showed visitor numbers actually rose 8.5% compared with the same time last year. However, Jason Wong Chun-tat, chairman of the Travel Industry Council of Hong Kong, told the South China Morning Post visitor numbers had been severely affected by the protests.

“The effects began to surface in June. Some individual travel agencies have reported a 30% to 50% increase in cancellations of mainland [Chinese] and south-east Asia travel groups to Hong Kong. The hotel and retail sectors expect a double-digit drop in tourist numbers in the second half of the year. The situation is worrying,” he told the SCMP.