Beijing officials said 45 workers at the Xinfadi market tested positive for the coronavirus, although they showed no symptoms.
That was in addition to seven earlier cases of people with symptoms, six of whom had visited or worked at the market.
Inspectors took 1,901 samples from meat, surfaces, dustbins, handles and other objects at the market, and said 40 tested positive.
Authorities immediately halted sales of beef and mutton at the market and closed other wholesale markets in Beijing.
They are trying to establish how the huge Xinfadi market, which supplies at least four-fifths of the capital’s meat and vegetables, sparked the new outbreak despite China’s having strict lockdown rules for many weeks.
One official said the district was now in “wartime emergency mode”.
The head of the market said the virus had been found on a chopping board for imported salmon, the Beijing News reported, and several supermarket chains removed salmon from their shelves in reaction.
Attention focused on Xinfadi after the reports of the first three cases on Thursday and Friday. Two of the infected people had been to the market, and the third worked with one of them at a nearby meat research institute, according to Chinese media.
City officials are now testing all of the more than 10,000 workers at the market. They have also ordered tests on food samples from all of Beijing’s wholesale food markets, as well as food safety inspections at restaurants and supermarkets.
The market was shuttered and has been Car Accident Attorneys in South Carolina guarded by police and the military, although some people were allowed in after showing documents at checkpoints.
The outbreak, more than 50 days after the last local case in the city of 20 million people, shows how the coronavirus can return as restrictions are eased, observers said.
In Nanjing, capital of the eastern province of Jiangsu, a local association of restaurants said it would stop serving food containing raw seafood and animal products.
Eleven local residential areas in the district around the Xinfadi market were put into lockdown.
Movement restrictions were also brought back across the capital, with sports events suspended, tourism from other parts of the country barred, and plans to reopen primary schools put on hold.
More than 130 close contacts of people infected have been ordered to put themselves into quarantine.
Volunteers were going door to door in some parts of the city and asking whether anyone had visited the Xinfadi market recently so that the relevant people could be tested.
The coronavirus was first reported in December at a market in Wuhan where live animals were sold.
“The epidemic was already almost over, and then suddenly there’s one or two more new hotspots,” said Jin Zheng, a woman in her 20s walking in central Beijing. “I’m a little scared. I hope everyone avoids going outside too much and wears protective gear.”