On Monday, the Beijing government said that 70 coronavirus cases had been found in Beijing since Friday. Nearly two-thirds have been in the district of Chaoyang, which ordered all 3.5 million residents to take three P.C.R. tests over the next five days. In other cities, mass testing in response to initial coronavirus cases has sometimes been a prelude to stringent lockdowns, like the four-week one in Shanghai that has kindled widespread complaints from residents there…

The outbreak in Beijing is another blow to the already stumbling Chinese economy. Waves of selling hit the Shanghai and Shenzhen stock markets on Monday, driving down by 5 percent in a single day the CSI 300 Index of large Chinese companies’ share prices.

Cases have been spreading in the community for a week, with multiple rounds of transmission, Pang Xinghuo, deputy director of the Center for Disease Control and Prevention in Beijing, said at a news conference on Sunday.