He underwent emergency surgery at Queen Mary Hospital in Pok Fu Lam on Thursday night and Friday morning, and his condition had improved from critical to serious as of publication time.

Sources said police were on full alert on the streets and had also stepped up cyber patrols, fearing copycat attacks with people already encouraging others on social media to target more officers.

Fully armed police in protective vests guarded the scene of the attack on Friday, reacting to calls online for people to gather there to “mourn” the assailant’s death and pay tribute to him.

Queen Mary Hospital rejected unsubstantiated rumours that some of its medical personnel allegedly sympathetic to the protest movement had been deliberately slow in providing emergency treatment to the policeman.

“The hospital emphasised that medical staff have always upheld professionalism and provided equal services to all patients. It is hoped that outsiders will not make unfounded remarks and stop forwarding false information,” a spokesman said.

Police on Friday also arrested a 24-year-old unemployed man in Ma On Shan in connection with the throwing of flammable objects early on Thursday morning near Government House, where Hong Kong’s leader lives.

Chief Executive Carrie Lam Cheng Yuet-ngor, having just returned from Beijing where she attended celebrations for the centenary of the Chinese Communist Party, strongly condemned the incident near her residence and the attack on the policeman.

“These show that even though the national security law after a year has turned Hong Kong from chaos to order … we still need to stay vigilant and think of danger in times of safety,” Lam said.

Newly promoted Secretary for Security Chris Tang Ping-keung, who visited the injured officer in hospital late on Thursday night, called the stabbing a “lone wolf-style act of domestic terrorism”.

Several pro-establishment political parties, as well as Law Society’s president Melissa Kaye Pang, issued statements to condemn the violence.

Police also hit out at internet users praising the attacker as a “martyr”, as some people visited the area outside Sogo to lay flowers in his memory.

In a statement on Facebook, a police spokesman strongly condemned acts that “glorified violence and provoked hatred”.

“We noticed that … internet users described the man as ‘valiant’, and paid tribute to him. This is blatantly supporting serious, violent and illegal acts,” the post read.

In another statement issued at 7pm, police said they were hunting for some radicals who had thrown a glass bottle onto the pavement at Percival Street in Causeway Bay. No injuries were reported.

The force also said later on Facebook that they had arrested a man in the district after finding a cutter and a Swiss army knife among his personal belongings.

On Friday evening, there was heavy police presence at the scene of the attack in Causeway Bay. A number of people holding flowers were stopped by officers. Some individuals were escorted to the side of the road for questioning or to be searched.

Asked if frontline officers were worried about their safety, Lam Chi-wai, chairman of the Junior Police Officers’ Association, said he was certain that the management would “make appropriate arrangements”.

“After this incident, I’m sure that the management will make serious considerations about the uniform, gear and weapons that we should use in our daily work, and in special operations. We have no doubt about that,” he said.

Speaking on condition of anonymity, a chief police inspector said he was not worried about his personal safety. “We will stay alert as usual. I just agree with the secretary for security that hatred is damaging society, and I am disappointed that some people can support such a criminal,” he told the Post.

In a press briefing on the arrest of the man accused of throwing flammable objects near Government House, acting chief superintendent Ho Chun-tung from the organised crime and triad bureau said it was shocking that people had romanticised and legitimised acts of violence.

“It is ridiculous that there are posts on the internet teaching people to engage in more lethal and cruel attacks … When people see these, they can be radicalised to engage in lone-wolf attacks, and it will end up hurting innocent residents and destroying the city’s peace and stability,” he said.

Social science experts warned against glorifying a suicidal attacker as a “martyr”, fearing that it could result in copycat crimes, but they also urged authorities to allow residents to vent their emotional stress using lawful means

A police insider said officers had raided the knifeman’s home in the Kowloon neighbourhood of San Po Kong soon after the attack, finding about five suicide notes addressed to different people and a large amount of what was described as propaganda material.

“The letters mentioned he planned to kill police on July 1 for Hong Kong. He told his parents to take care of his affairs after he died,” the source said.

The suicide notes also mentioned his hatred of police, his negative feelings about the national security law and other social issues, according to the insider.

The assailant, who was described as a recluse and introvert, was single and lived with his parents, who apparently did not even know where he worked. No documented history of mental illness has been established.

Officers seized his computer and were investigating whether he had links to any political groups.

The source stressed that Hong Kong’s terrorist threat level remained “moderate”, but officers had stepped up patrols in particular locations, especially around Government House.

“Labelling the case as a terrorist attack should send a strong and clear message to the public that such an attack is serious and could happen anytime. People should be mindful,” the source said.

The source added that officers had stopped and searched people at the crime scene soon after it was cordoned off, arresting an 18-year-old girl with a knife in her possession. Police were still investigating whether she had any connection to the attack.

Security chief Tang is expected to attend a scheduled, regular Legislative Council panel meeting next Tuesday to talk about domestic terrorism.

Lai Tung-kwok, Hong Kong’s security chief from 2012 to 2017, expected Tang to reassure the public about the government’s anti-terrorism strategy, while pro-establishment lawmaker Gary Chan Hak-kan said his colleagues would be asking Tang to elaborate on how the police, customs and immigration departments would work together to guard the city against such “lone-wolf” attacks.

Additional reporting by Danny Lee