Millions were stuck at home in China's Chengdu on Friday after a handful of COVID-19 cases brought the megacity to a standstill.

Supermarket shelves were stripped bare this week as locals in the city — a powerhouse economic hub in China's southwest that is home to 21 million people — feared a repeat of the months-long lockdown in the eastern megalopolis of Shanghai earlier this year.

Long lines of residents queued for mandatory testing, while videos verified by AFP showed supermarket shelves cleared of produce.

One local, who asked to remain anonymous, told AFP he believed "everyone was crazily stocking up for goods" because of the experience of Shanghai, which was hit by food shortages during its lockdown.

The 25-year-old said he had been in the eastern city during its shutdown and had since been "habitually stocking up" before Chengdu's latest measures were announced.

Under the rules, in force until Sunday, each household will be allowed to send one person out to buy groceries and essential goods per day, provided they have tested negative for COVID-19 in the previous 24 hours, an official notice said.

It added that all residents would be tested for the virus, urging them not to leave the city unless "absolutely necessary".

The mood appeared calmer on social media Friday, with some residents saying they were able to order food to be delivered to their apartment gates and to go out to buy groceries.

Others said they had resorted to sleeping at their offices in order not to miss work.

Authorities had initially sought to quash talk of a looming lockdown, with police saying they had detained a man for "creating panic" after he warned that the city could shut down.

His case drew online attention Friday, with many on the Twitter-like Weibo platform questioning his punishment and calling him a "hero" for warning his fellow citizens.

Authorities have ordered multiple rounds of mass testing between Thursday and Sunday, with the city reporting 150 local COVID-19 infections on Friday, 47 of which displayed no symptoms.

China is the last major economy wedded to a zero-Covid policy, stamping out virus flare-ups with snap shutdowns, mass testing and lengthy quarantines.

That has proved increasingly challenging since the emergence of the fast-spreading Omicron strain, with all of China's mainland provinces reporting local infections over the past 10 days.

Five districts in the southern tech hub of Shenzhen closed bars and cinemas on Thursday, with rumors of a citywide lockdown prompting a run on online grocery apps.

Last month, travelers in the southern island province of Hainan protested after more than 80,000 tourists were stranded in a resort city because of a COVID-19 flare-up.