As the procession reached the vicinity of the vast factory of the Tunisian Chemical Group, a public company, police fired large amounts of tear gas. Hundreds of people retreated, but groups of young people remained shouting their anger, while several individuals fainted, according to an AFP correspondent on site.

In recent weeks scores of people have been hospitalised in the city of Gabes, with residents pointing the finger at the potentially cancer-causing waste from a phosphate processing plant nearby.

"This has to stop. My three kids and I are asthmatic, my husband and my mother died from cancer as a result" of the plant, 52-year-old protester Lamia Ben Mohamed told AFP.

"We want to breathe," the protesters chanted, while dozens of motorcycles at the head of the rally honked their horns.

According to an AFP journalist at the scene and police sources, the crowd's size began at around 2,000 people before growing to several thousand.

Organised by the Stop Pollution collective, the rally demanded the shuttering of the ageing fertiliser plant, whose discharges into the Mediterranean Sea have long sown discontent among Gabes residents.

They blame the plant for collapsed fishing stocks, beach pollution, respiratory diseases and cancer.

That outcry has intensified in the past month. The rally comes a day after 122 people had to be treated or hospitalised for cases blamed on the plant, according to a local official with knowledge of the figures.

Marwa Salah, 33, a cardiologist at Gabes Regional Hospital, said she wanted to "live without the pollution from the complex that has brought us nothing".

Wrapped in the Tunisian flag or holding yellow banners bearing a skull, protesters carried signs reading "Stop genocide", "Gabes without oxygen", and "The complex is killing us under the state's watch".

According to Slah Ben Hamed, regional leader of the UGTT union, the recent waves of poisoning were caused by "outdated equipment" and "gas leaks".

Fertiliser production requires treating phosphates with sulphuric acid and ammonia.

Although the Tunisian state had promised in 2017 to begin the plant's gradual closure, authorities earlier this year said they would ramp up production instead.

Experts have cast doubt on the possibility of cleaning up a complex first inaugurated in 1972.