A mass Covid-19 testing order in Beijing's biggest district prompted residents in the Chinese capital to stock up on groceries, fearing they could be destined for a lockdown similar to that of Shanghai, which entered the fourth week of bitter isolation.

Authorities in Chaoyang, home to 3.45 million people, late on Sunday ordered those who live and work there to be tested three times this week as Beijing warned the virus had "stealthily" spread for about a week before being detected.

Knowing how Shanghai residents struggled to source food and other essentials while locked indoors, shoppers in Beijing crowded stores and online platforms to stock up on...