Shanghai, China, canceled over 500 flights and locked down several residential areas over three COVID-19 cases.
Twenty hospitals also suspended outpatient and emergency medical services for three days.
The sweeping response is part of China's strict COVID zero tolerance policy.
Chinese financial hub Shanghai canceled hundreds of flights, closed schools, and suspended tourism programs on Friday after three locally-transmitted COVID-19 cases emerged in the city.
According to a municipal government statement, the three positive cases were friends from the nearby city of Suzhou who attended a lecture about ancient architecture last week. They were in close contact with almost 200 people, prompting 20 hospitals to suspend outpatient and emergency medical services for three days out of caution, per the statement.
All three of the travelers were fully vaccinated, authorities also said.
The restrictions are part of Beijing's COVID-zero policy: China's attempt to completely stamp out any trace of the coronavirus within its borders through swift lockdowns and curbs.
Around 30% of the flights from two of Shanghai's major airports were canceled following the discovery of the infections on Thursday, per local media outlet Jiemian News.
The Guardian estimated more than 500 flights were canceled, citing data from the flight tracking site VariFlight.
Shanghai authorities also told travel agencies to suspend all city tours or travel programs to other provinces, local outlet Yicai Global reported.
The city, which has a population of 24.9 million residents, also locked down several residential compounds believed to have a higher infection risk, Reuters reported.
"China has accumulated lots of epidemic prevention experience in 'dynamic COVID-zero,' so our strategy won't change," said Zhang Wenhong, the head of Shanghai's pandemic prevention task force, in a media briefing on Thursday.
"In the future, this will become a normalized epidemic prevention scene in the autumn and winter. Similar situations will happen in the future," said Zhang, adding that the city's focus now is to reduce its pandemic response's disruption to residents' lives.
The last time Shanghai discovered a domestically-transmitted COVID-19 case was when it closed the city's Disneyland after a case was discovered Halloween night. More than 30,000 visitors were trapped in the park and underwent COVID testing before being allowed to leave.