Chinese police have detained 18 people over large rallies opposing a chemical plant in Maoming, state-run media reported Thursday, as locals said the protests had entered a fifth day.

Thousands of demonstrators have taken to the streets of Maoming, in the southern province of Guangdong, for days of protests, some of them met with violence from police, locals told AFP.

Maoming's city government said it had detained 18 people for "assembling a crowd to disturb public order", the state-run People's Net reported, citing local authorities.

The charge is often used to detain protesters in China.

The Maoming residents have been rallying against a plant producing paraxylene (PX), a chemical used to make fabrics. Proposals for PX plants, often perceived as a health risk, have sparked large protests in several other Chinese cities in recent years.

Photos and video posted on social media websites this week, which could not be verified, showed hundreds of people holding placards with messages opposing the plant.

They also showed police officers beating protesters, as well as cars and shop fronts left in flames.

One Maoming resident who declined to be named told AFP that police had fired rubber bullets towards protesters. City officials denied claims posted online that several people had been killed.

More than 10,000 police have been deployed in Maoming during the rallies, a local resident surnamed Dong told AFP.

Hundreds resumed protesting on Thursday, he said, but without further violence.

"Since the leaders were arrested, things have become quiet," he said.

Information about the protests is hard to verify, with online reports swiftly censored. But they sparked smaller rallies in other cities in Guangdong, according to reports.

Tens of thousands of protests are thought to occur across China every year, but are tightly controlled by the ruling Communist party, which is wary of any social unrest.

There have been an increasing number of large-scale environmental protests in recent years, especially in urban areas.

Local authorities in the coastal city of Xiamen cancelled plans for a PX plant after thousands took part in a protest in 2007.

A huge protest in the northeastern city of Dalian in 2011 prompted authorities to announce the closure of another factory, although it was apparently still operating two years later.