Millions in Beijing's largest district have taken their second Covid-19 tests this week as the Chinese capital tried to keep an outbreak of dozens from spiralling into a crisis that could force it into a distressing Shanghai-type lockdown.

In Beijing, supermarkets have kept food and other supplies well-stocked under orders from authorities, Reuters reported.

Local officials also hoped early mass testing would spare them the drama in Shanghai, where officials waited for about a month as cases spiralled before ordering a city-wide screening campaign.

Beijing was testing the more than 3.5 million residents of its Chaoyang district on Wednesday, all of whom were screened on Monday as well. On Tuesday, 16 million from other districts were also tested and are due for another round on Thursday.