When the meeting broke up without agreement shortly before 3pm, the woman is said to have pulled out a semi-automatic Beretta 950 handgun and gunned down the siblings at close range, before calmly walking away.

Tsim allegedly shot her 80-year-old aunt in the front of the head and her 62-year-old uncle in the back of the head. Both were rushed to Pamela Youde Nethersole Eastern Hospital in Chai Wan, where the aunt died at 6.35pm. The 62-year-old uncle remained in critical condition on Wednesday.

The two other victims, a 72-year-old uncle and a 60-year-old aunt, were shot in the shoulder and hand and taken to hospital, with the uncle’s condition improving from critical to serious, while the aunt remained stable, a government spokesman said on Wednesday.

“The four victims were shot as they were about to leave,” a police source said.

Officers from the Counter Terrorism Response Unit who were on patrol nearby raced to the scene, and with help from security guards at the nearby Cityplaza shopping centre, arrested Tsim, who was still carrying the gun, along with about 40 rounds of ammunition.

Initial information indicated that one of the Citiplaza guards, a retired policeman, intercepted Tsim before the officers from the terrorism unit arrived.

Tsim is the daughter of a fifth sibling, who was the eldest sister of the group and recently died. A source said that family members told police that she worked as a bodyguard in mainland China.

“In the incident, at least four shots were fired, according to our preliminary investigations,” said Chief Inspector Hui Kong-kit. “We will investigate how the assailant obtained the firearm to commit the crime.”

The source said that police had not ruled out the possibility the handgun and ammunition were bought online and airmailed into Hong Kong.

In the evening, officers scouted around the park to search for spent cartridges to try and establish how many shots were fired.

Hui said shells had been recovered from the scene, and emphasised that the gun was not a police-issued firearm.

According to the chief inspector, the suspect was calm when arrested and did not have any mental health problems, although he would not say if she had a criminal record.

The victims and suspected shooter were relatives and the motive was a “family dispute”, Hui said. They had arranged to meet each other in the park before the incident, he added.

“Two of them were injured in the head, and the other two were shot in the shoulder and hand,” a police source said, adding it was not a “random shooting”.

“A witness told officers he heard the sound of gunshots and found two injured people in the park,” a police spokesman said.

Meanwhile, the park was put on lockdown as officers from the Police Tactical Unit arrived to investigate the incident.

Forming a line, police swept the area for evidence as residents and office workers in the usually quiet middle-class enclave gathered to watch.

Several police cars, ambulances and fire engines lined Tai Koo Wan Road.

A small area of the Cityplaza Four was also closed off as a command post. The suspect was seen sitting inside, wearing a hood as detectives worked on the early stages of the case.

One Quarry Bay resident said she was worried about her grandchildren, who play in the park every day.

“This is unbelievable,” she said, as a security guard ushered her towards another route around the park.

“I was told that there were gunshots and to come here immediately,” said one security guard, standing outside one of the park’s entrances.

At least one construction worker the South China Morning Post spoke to said he had heard “a few gunshots” ring out in the area as he worked near the park.

But the reaction of most residents and workers in the area was mostly shock and disbelief, not least because Tai Koo was known to be a safe neighbourhood.

Tai Koo resident Catherine Yu, 37, a mother of one, said: “It is unusual to have these incidents happen here [in Hong Kong].”

Luna Lindsay, 42, an American who works in Cityplaza, said she was shocked to hear about any sort of gun violence in Hong Kong.

She came to Hong Kong two years ago with her family.

“I was surprised ...I thought it is hard to obtain firearms in Hong Kong unlike where I’m from.”

Fujita Yuka, a housewife and mother of two, said she found the incident unbelievable.

“I came to Hong Kong with my husband almost 10 years ago, this is the first time something like this happened near my home. I’m a bit worried for the safety of my kids, but I think it won’t be a big problem.”

Carmen Yip, a Tai Koo resident and mother of two, said she was less concerned about the reason behind the shooting but why and how a woman could obtain a firearm and carry it around Hong Kong.

At the scene, police including officers from the Counter Terrorism Response Unit wearing helmets, and carrying MP5 submachine guns were seen standing guard.

In response to global terror attacks, Hong Kong anti-terrorism officers started to patrol the city’s railway network in late 2016.

The Post reported in September last year that the number of sensitive locations the elite officers were inspecting and patrolling had increased threefold to 600 over the past eight years.

The last time a gun was fired during a crime in the city was in 2015, when an armed robber shot a Tsim Sha Tsui shop assistant and made off with nine Patek Philippe watches valued at HK$5.5 million (US$700,000).

In 2006, three suspected triad members were wounded in a predawn shoot-out in Tsim Sha Tsui when they were among a group of five who had just left a private club in Austin Avenue after watching the World Cup final. Police said they were ambushed by two armed men. One of them fired four shots from a shotgun.