(Adds comments from Chengdu resident, general call by health authority to avoid travel during upcoming holidays)

By Ryan Woo and Roxanne Liu

BEIJING, Sept 8 (Reuters) - The Chinese city of Chengdu extended a lockdown for a majority of its more than 21 million residents on Thursday to prevent further transmission of COVID-19 while millions more in other parts of China were told to shun travel in upcoming holidays.

Chengdu, the capital of southwestern China's Sichuan province, was locked down on Sept. 1 after COVID cases were detected, becoming the largest Chinese metropolis hit with curbs since Shanghai's lockdown in April and May.

Since last week, Chengdu has mostly reported fewer than 200 new infections a day, a tiny fraction compared with outbreaks in other parts of the world. It found 116 new cases for Sept. 7 versus 121 the day earlier, authorities said on Thursday.

The lockdown had been expected to be lifted on Wednesday but officials said late in the day that the virus still posed a risk in some areas and extended the lockdown for 16 million of the city's residents.

Those under lockdown will be tested every day and anyone who tests positive will be quarantined. Residents in areas deemed high-risk are not allowed out of their homes.

Chengdu is aiming for zero new community infections within a week, its COVID prevention and control command said in a statement.

The extension puts a further strain on the city, which recently has also endured heatwaves, power cuts and an earthquake.

"I'm concerned the lockdown will be further extended," said Zhang Yue, a Chengdu resident.

"Many people aren't taking it seriously. In some neighbourhoods, people secretly meet up to play mah-jong, while some refuse to do the tests scheduled early in the morning."

'MINIMISE TRAVEL'

China has been battling to contain the highly transmissible Omicron variant, imposing lockdowns of various degrees to stop its spread. Shanghai was locked down in April and May while Xian, Shenzhen and Guiyang have also undergone lockdowns and restrictions.

Guizhou-Cloud Big Data Industry, a state-owned firm that manages Apple's cloud services data centre in Guiyang, said in a statement that many employees were living and working on site, away from their families.

"Every employee sees this chance to participate in this great war as his honor and pride!" the company said on Wednesday.

Japanese brokerage and investment bank Nomura said that, as of Tuesday, 49 Chinese cities had various levels of lockdowns or control measures, with an estimated 291.7 million people affected, up from 161.3 million the previous week.