Hong Kong officials were making frantic efforts to track down anyone who had contact with a Mexican who tested positive for swine flu, with the government calling it a "critical moment."
The city was placed on its highest health alert after the 25-year-old man tested positive for the A(H1N1) virus on Friday, the first confirmed case of swine flu in Asia.
As some pharmacies sold out of protective masks, more than 300 guests and staff at a hotel where the Mexican stayed were put in isolation for seven days.
Officials were Saturday desperately trying to reach anyone who may have come into contact with the infected man during his journey from Mexico, via Shanghai.
"This is a critical moment," Hong Kong's Secretary for Food and Health, York Chow, told legislators.
In particular, officials were trying to trace the 142 fellow passengers on China Eastern Airlines flight MU 505 from Shanghai and the two taxi drivers who drove the Mexican, who has not been named, after he arrived in Hong Kong.
"If (the taxi drivers) don't appear and if they have already got infected, the consequence will be unimaginable," Chow said, adding they were liaising with police and taxi drivers' associations.
But he also said many of the tough measures were precautionary and residents should not panic.
"At this stage, there is no sign of a potential outbreak in our community," he said.
The Mexican man was admitted to hospital in Hong Kong on Thursday night suffering from a fever and tested positive on Friday for the flu virus. He is in a stable condition, authorities have said.
Two travelling companions and a Hong Kong friend have also been placed in quarantine.
China said Saturday it was to put all passengers on the flight from Mexico to Shanghai which the man was on under quarantine, after suspending all flights from the South American country on Friday.
Hong Kong's shutdown of the Metropark Hotel in the busy bar district of Wanchai has drawn criticism.
"It is stupid to lock up the hotel, as most people in the hotel did not contact (the infected man) at all," said infectious diseases expect Lo Wing-lok, according to the South China Morning Post.
"I believe this is going to be an international joke... overseas, healthy tourists are going to be locked up in Hong Kong for seven days."
The government said Saturday that 12 of the hotel guests had refused to remain at the hotel and had been moved to a holiday camp which has been specially designated as an isolation zone.
Despite putting Hong Kong on its highest level of alert, chief executive Donald Tsang said Friday all social activities and exhibitions would go ahead as normal, including the annual Bun Festival which normally draws huge crowds.
"I stress we don't need to panic," he told reporters late Friday.
Chow said a decision on whether to close all the city's schools would be announced by the end of Sunday, but it was unlikely at this stage.
Memories of the deadly SARS virus in 2003, which killed close to 300 people in Hong Kong, help explain the city's tough measures, which even Chow has described as "draconian."
Hong Kong, which also remains on alert for bird flu, became a virtual ghost town when the SARS virus hit, and residents have been quick to step up hygiene measures since swine flu began to spread around the globe.
Some pharmacies have sold out of protective face masks in recent days, and the masks have become much more common on public transport and in restaurants, although the Central bar district remained lively on Friday night.
Passengers on a flight from Taipei put on protective masks as a plane landed at Hong Kong's International Airport Saturday morning, and almost all airport staff were wearing the masks.
Before the confirmed case, Hong Kong had previously announced a series of tough measures, including introducing the power to detain anyone showing symptoms of the virus after arriving from an infected area.
In South Korea, authorities Saturday confirmed the second case in Asia, while Denmark and France have joined the list of countries who have detected the virus.
Mexico's government has confirmed 16 people have died from swine flu, with 381 infected.