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, according to uncorroborated posts on Twitter and Chinese social networking sites.

The protesters gathered in front of the provincial government headquarters at the intersection of Zhengyi Road and Renmin Road at about 10am, according to the posts. The demonstration has drawn a large police presence and began with one arrest, but has remained largely peaceful.

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, 17 miles (28km) south-west of the city centre.

Every year the refinery would produce 500,000 tons of paraxylene (PX), a carcinogenic chemical used in production of polyester, according to the state-run China Daily newspaper.

Thursday's demonstrators donned face masks displaying anti-PX messages, shouted "roll out, protest!" and sang the national anthem in unison, according to Twitter reports.

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.

"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 before Thursday's protest.

"If the government clings to its position, the public's resistance will only increase."

According to pictures posted on the popular Chinese microblogging website Sina Weibo, protesters held banners reading: "Save Kunming! Help us! We love Kunming, oppose pollution" and in English, "Save the water for the life!" The pictures have since been deleted, and searches for Kunming PX have been blocked.

Many university students in Kunming have been blocked from leaving their campuses, according to reports online. On Saturday the municipal government sent text messages to Kunming residents claiming that the project "will not produce PX".

Many Kunming residents appear unconvinced. "If the refinery is [as] clean and safe they claim it to be, why does the government not dare to publish the environmental review report," a demonstrator told the South China Morning Post.

A similar protest earlier this month in Chengdu, the capital of adjacent Sichuan province, was suppressed by police.

Environmental protests have become more common in recent years, as many Chinese people become increasingly exasperated by the government's growth-first development strategy and lack of transparency.

A Shanghai battery manufacturer announced on Wednesday that it would cancel plans for a new plant after hundreds of people staged three protests to voice concerns about its possible environmental impact.

In August 2011 a protest in the north-eastern city Dalian led local authorities to announce that they were would relocate a polluting PX plant. The following summer, the coastal city Qidong scrapped a pipeline plan after about a thousand protesters stormed government offices and overturned cars.