Of the 38 locally transmitted cases, 36 were in the capital, Beijing, where the authorities are conducting mass testing at a major seafood and produce market that appears to be the source of a new outbreak. It is the most cases the city has reported in one day since the coronavirus first emerged. Beijing had gone eight weeks without a single locally transmitted case until a total of seven were detected on Thursday and Friday.

The other 19 cases China reported on Sunday involved travelers arriving from overseas, mostly in the southern province of Guangdong.

Nearly all of the dozens of people who tested positive in Beijing in recent days had worked or shopped at the Xinfadi market, a wholesale market on the city’s south side that sells seafood, fruit and vegetables, according to the Beijing health commission. The market has been shut down and several nearby residential complexes are on lockdown.