Thousands of protesters have gathered in the southern Chinese city of Kunming for the second time this month to voice concerns over the environmental impact of a planned chemical plant, underscoring the increasing willingness of China’s emerging middle class to challenge government decisions by taking to the streets.

Around 2,000 protesters gathered in front of the Yunnan provincial government headquarters in a demonstration which drew a large police presence and began with one arrest, but remained largely non-violent.

Kunming’s first environmental protest this month was held, without arrests, on 4 May, after China National Petroleum Corporation announced plans to build the chemical plant in Anning, a county seat 18 miles south-west of the city centre. Every year, the refinery would produce 500,000 tonnes of the chemical paraxylene (PX), a suspected carcinogen used in production of polyester, according to the state-run China Daily newspaper. Kunming’s municipal government has denied the claim, but residents fear the city’s air and water will be polluted.

“We don’t need speedy development. What we need is a healthy and peaceful country,” a Kunming resident, Liu Yuncheng, told the Associated Press. “I still haven’t given birth to a baby. I want to be pregnant and I want a healthy baby.”

China’s nimby demonstrations have proliferated in recent years, as its affluent, educated, and tech-savvy rising middle class grows exasperated with the government’s “growth-first” development model and shadowy decision-making process.

The risks are high – the Chinese government strictly forbids most public protests, and crackdowns on similar demonstrations have been severe. Another “anti-PX” protest earlier this month in Chengdu, the capital of adjacent Sichuan province, was pre-empted by the arrival of hundreds of anti-riot and paramilitary police.

Yet experts say that if protesters refrain from challenging the Communist party’s grip on power – and if the potential costs of cracking down outstrip those of ceding to public demand – authorities may tread with caution.

Chinese people have a “much higher sense of environmental rights” than they have in the past, said Ma Jun, the CEO of the Institute of Public and Environmental Affairs in Beijing.

Ma added that Sichuan authorities may have clamped down because the PX plant near Chengdu was already nearing completion. “To my knowledge [the Kunming project] is in an earlier phase, and I think there are still opportunities there for reconsideration,” he said. Furthermore, the province is still reeling from a devastating earthquake last month, raising the risk that a mass demonstration could spin out of control.

Thursday’s demonstrators donned face masks displaying “anti-PX” messages, shouted: “Roll out, protest!” and sang the national anthem in unison, according to firsthand reports on Twitter. Photos posted online show a thick line of police pressed tightly against rows of protesters, many of them documenting the standoff with smartphones and digital cameras.

“We cherish blue skies and white clouds, as well as good air. If you want to build a refinery with 10m tonnes of capacity here in the place where we live, we resolutely oppose it,” said a Kunming resident who identified herself only by her surname, Liu. “We want a good life. We women want to be beautiful,” she told the Associated Press.

At about 4pm, Kunming’s mayor, Li Wenrong, addressed the crowd in a seemingly impromptu press conference. Li promised “equal dialogue”, public hearings about the refinery’s future, and increased investment in pollution control, the Hong-Kong-based South China Morning Post newspaper reported, adding that some onlookers dismissed his vows as lip service.

Li parried censorship-related queries by saying: “All levels of government have good intentions … but their methods may not always be right,” the newspaper reported.

“Protest activities only happen on the precondition that the government doesn’t offer opportunities for information transparency, dialogue and negotiation,” said an influential Kunming-based blogger who uses the name Bianmin, or “frontier person”, in an email interview. “If the government clings to its position, the public’s resistance will only increase.”

Concessions to such protests by local governments and state-owned companies are not without precedent. A Shanghai battery manufacturer announced on Wednesday that it would cancel plans for a new plant after hundreds of locals staged three successive protests about its potential environmental impact.

In August 2011, a massive protest in the north-eastern city of Dalian led local authorities to announce that they would relocate a polluting PX plant. Last autumn, authorities in Ningbo City, in coastal Zhejiang province, scrapped plans to expand a similar state-owned plant after a week-long demonstration by thousands of aggrieved residents.

Li Bo, head of the Beijing-based NGO Friends of Nature, said the recurrence of “anti-PX” demonstrations showed that China’s environmental authorities had been slow to learn from past mistakes.

“There were various attempts to find out more about this project by citizens in Kunming, but they have had a lot of difficulty getting satisfactory information,” Li said. “A lot of worries and doubts have accumulated, which is more or less what happened with the previous PX projects in Dalian and Ningbo.”