The Chinese port city of Tianjin is aiming to test its 14 million residents within the next 48 hours after the discovery of a cluster of Covid cases.
Residents have been advised to stay home until tested, and must return a negative result to get the health pass needed to use public transport.
Of the 20 cases discovered, two are the highly transmissible Omicron variant.
China is pursuing a zero-Covid policy, which aims to eradicate the disease in the community.
In contrast to other parts of the world, which have opened up following vaccination campaigns, China has responded to small numbers of local cases with mass testing and tight lockdowns.
The policy is likely to come under pressure with Lunar New Year on 1 February, when millions of people normally travel, as well as the Winter Olympics starting a few days later. Host city Beijing is just 115 km (70 miles) from Tianjin.