China reported over 20,000 Covid-19 cases on Wednesday, the highest daily tally given since the start of the pandemic, with Shanghai the heart of the virus surge despite being in lockdown.
The country's "zero-Covid" strategy has come under immense strain as cases spike.
Until March, China had kept daily cases low with snap localized lockdowns, mass testing, and strict restrictions on international travel.
But the caseload has hit thousands per day in recent weeks, with officials saying they have detected a mutation of the highly transmissible Omicron variant near Shanghai.
China recorded 20,472 infections on Wednesday, the National Health Commission said in a statement, adding that there were "no new deaths."
It is the country's highest-ever daily infection number given by authorities, even during the peak of the initial outbreak which centered around Wuhan.
The majority of the cases are, however, asymptomatic, AFP reported.
Yet in Shanghai quarantine facilities are bulging with people who test positive as city officials stick rigidly to virus protocols.
Those include separating Covid-positive babies and children from parents who test negative, a policy that has stirred anxiety and anguish from worried families.
Shanghai, China's largest city, accounted for more than 80 percent of the national tally, city officials said on Wednesday.
The metropolis of 25 million locked down its residents in phases last week, prompting scenes of panic-buying and mass testing.
A top Shanghai official has conceded that the financial hub had been "insufficiently prepared" for the outbreak.