The districts in Guangzhou subject to mass testing this week include Haizhu, which has seen the bulk of the city's cases.
Lily Li, a Guangzhou resident, said the outbreak in the city had worsened in the past two days, having spread to Tianhe, just north of Haizhu.
While COVID cases in China are small by global standards, the policy response has been relentless and mass testing for large populations has been the norm since 2020.
Mass testing is generally free, but some local governments are resuming charges for tests as their finances come under strain amid a slowing economy.
A COVID testing company in Xuchang, a city in Henan province, said on Tuesday they would stop all testing-related work from Friday due to late payments from authorities.