Thousands have recently rallied in the city of some 400,000 inhabitants to demand the closure of a state-run phosphate processing plant which they say is behind a rise in gas poisonings after it ramped up production.

The plant, inaugurated in 1972, processes phosphate to make fertilisers, and some of the gases and waste it discharges into the open air and Mediterranean are radioactive and can cause cancer, researchers have found.

"Everything is closed in Gabes," said Saoussen Nouisser, a local representative of Tunisia's main labour union, UGTT, which called the general strike.

"We're all angry at the catastrophic environmental situation in our marginalised city," she told AFP.

Thousands marched in the city centre in the afternoon, calling for the factory's closure.

"Health is a right, breathing is a right," protesters chanted. "Living without pollution is a right."

Activist Noureddine Boukhris said the protesters would ramp up their campaign after the general strike.

"We want the state, the prime minister's office and the ministers involved in this matter to react by dismantling the plant," he said.

- 'Urgent measures' -

Over 200 people have been hospitalised in recent weeks for respiratory distress and gas poisoning, according to authorities and NGOs.

Videos circulated online of children seemingly having breathing issues last week pushed thousands to rally in the coastal city.

Police have used tear gas to disperse the crowd at some demonstrations, with the city seeing clashes between residents and security forces sometimes at night.

Dozens were arrested over the weekend.

National Guard spokesman Houcem Eddine Jebabli told local media that protesters have used "over 800 Molotov cocktails" targeting security forces, adding there were cases of "robbery and looting".

In 2017, the Tunisian government promised to gradually shut down the factory, but earlier this year authorities said they would instead ramp up production at the plant.

President Kais Saied had long vowed to revive the sector hindered by unrest and underinvestment, calling phosphate a "pillar of the national economy".

Authorities have said "urgent measures" were underway to address pollution in Gabes as the government called on Chinese companies to help manage the plant's waste.

Equipment Minister Salah Zouari said they would help "control gas emissions" and "prevent the discharge of phosphogypsum into the sea".

The solid waste contains radioactive elements, which also impact soil quality and groundwater.

Taking advantage of rising world fertiliser prices, the government now wants the plant's output to nearly quintuple by 2030, from less than three million tonnes a year to 14 million tonnes.