Guangzhou has placed an area under lockdown after the city reported its first Omicron coronavirus infection.
The case involved a man, 67, who returned from overseas late last month and repeatedly tested negative for Covid-19 during quarantine.
He returned to his home in Yuexiu district on Saturday and remained in isolation.
Tests on Sunday showed he was carrying the Omicron variant.
Authorities have sent people living in his building to a quarantine centre for medical surveillance.
Residents in the area have been banned from leaving their homes and required to get tested twice in three days.
Officials have appealed to residents to comply with the testing and control measures.
The Guangzhou Baiyun International Airport says it will strengthen checks on the health codes of departing travellers.
Dr Siddharth Sridhar, a microbiologist from the University of Hong Kong, said there's no need for undue alarm here.
"I think we have a very strong mechanism in place to detect cases of Covid-19, not just Omicron but any variant, in our returning travellers under the current zero Covid-19 policy.
"So I think we just have to remain vigilant and I won't say there's cause for undue panic at this moment."
Separately, the Guangdong city of Dongguan has reported two new Covid-19 infections.
Authorities there said both patients had travelled to Shenzhen and were asymptomatic.
Last updated: 2021-12-14 HKT 18:32