The city of Guangzhou on Wednesday imposed Covid-19 curbs in parts of the city, joining Shenzhen in battling local flare-ups.

Several of the mainland's biggest cities have stepped up Covid-19 restrictions this week, affecting the activities of tens of millions of people. Mindful of balancing economic needs with efforts to contain each and every outbreak, authorities said the curbs would last for just a few days, although some smaller cities extended restrictions earlier this month.

Guangzhou, a city of nearly 19 million residents near Hong Kong, reported five locally transmitted infections for Tuesday but authorities ordered certain areas in one district to close indoor entertainment venues and dining at restaurants until Saturday.

Health authorities in Guangzhou said the infected people had been to multiple venues.

"This outbreak was caused by the Omicron variant BA.2.76. We're conducting epidemiological investigations day and night," deputy director of the Guangzhou Health Commission Zhang Yi said.

"The source of infection is not yet clear, but judging from the current epidemiological trajectory, the patients had been to many venues, including crowded places such as swimming pools and markets, and there is a high risk of community transmission," she added.

The city also ordered all kindergartens and primary, middle and high schools in the district to delay resuming fall semesters and halt offline sessions that have already started, according to state media reports on Wednesday. Bus and subway services in the district were also reduced.

In Shenzhen, at least four districts with around 9 million residents in total, have already ordered the closure of entertainment and cultural businesses and halted or reduced restaurant dining for a few days.

The combined economic output of Shenzhen and Guangzhou reached 5.89 trillion yuan last year, equal to about half of South Korea's gross domestic product.

China reported 1,675 new domestically transmitted Covid-19 infections for August 30, the National Health Commission said on Wednesday, compared with 1,717 new local cases a day earlier.

Most cases were found in the region of Tibet and the province of Sichuan.

The commercial hub of Shanghai would tighten control over large gatherings and avoid unnecessary ones, a municipality official said, as the city reported two new local infections on Tuesday, including one found in the community.

Large conferences and events involving travel across cities and regions must be downsized, delayed or moved online whenever possible, Wu Qianyu, a Shanghai health official, told a news briefing. (Additional reporting by Reuters)