An environmental protest in China was aggressively put down at the weekend, with a large police presence continuing for days to prevent further demonstrations in an unusually heavy-handed response.

The protests started after the south-western city of Chengdu was shrouded in thick smog, with some residents placing pollution masks on statues. An unknown number were taken away by police, with security forces in riot gear seen in the city’s downtown shopping area.

Toxic clouds of smog are regular features of China’s major cities, with some studies showing pollution has caused about 1 million premature deaths a year. China’s leaders have declared a “war on pollution”, but many state-owned companies are leaders in smog-producing industries such as steel, coal and power.

Authorities have been worried for years that the deadly air could spark protest. A 2015 film focusing on the country’s pollution and its effect on humans was quickly censored after being watched hundreds of millions of times online.

Tianfu Square, in the heart of the city, was still closed on Monday, with police cars parked in the middle and officers at the edges preventing people from entering. Police ordered copy shops to record details of anyone seeking to photocopy flyers complaining about the smog and to contact the authorities, according to photos of an official notice circulating online.

Anyone placing large orders of face masks should also be reported, the notice said.

Police rarely allow protests in China, but small-scale demonstrations are usually dispersed quickly and without mass detentions. The response in Chengdu was severe given the size, although exact numbers for the protest are unknown.

Some reported being stopped and questioned by police simply for wearing pollution masks in the same neighbourhood as the demonstration, according to social media posts, with at least a dozen being detained.

Residents quickly took to social media, calling for more action and posting photos of themselves with signs saying “let me breathe”.

“We won’t put up with this! Take to the streets! We are all guilty of producing a world like this. Come on, kids, let’s stay alive!” wrote one commenter according to Radio Free Asia.

Police detained one man for sharing photos that was purported to show a huge crowd protesting the smog, but was actually taken in 2012.

Chengdu, the capital of southwestern Sichuan province, lacks many of the heavy industries that cause the capital Beijing to issues “red alerts” for pollution.

But the air is far from safe.

Levels of PM2.5, fine particulate matter that penetrates deep into the lungs, was more than six times the World Health Organization recommended yearly average in the first half of 2016, according to data compiled by Greenpeace.