Beijing identified three more neighborhoods on Wednesday as high-risk and four more as medium-risk, a label that prompts lockdowns.

The scale of Beijing’s testing program is enormous. Folding tables have been set up every few blocks near large roads. Up to a half-dozen volunteers help to direct residents and take their names, and several medical workers perform the swift oral swabs.

Using so-called batch testing, numerous swabs are placed in pink liquid in a single test tube and sent to a lab. If the liquid tests positive for the presence of the virus, then the authorities contact each person who had a swab in the tube and conduct intensive nasal P.C.R. testing.

Mr. Li said that the city had mobilized 35,000 medical workers who take the samples and another 104,000 volunteers and other auxiliaries to help them.