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 plant, whose discharges into the Mediterranean Sea have long sown discontent among Gabes residents.

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.

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 sanitising a complex first inaugurated in 1972.