Some were carrying children in panic into a hospital in the southern city, an AFP reporter saw, after fresh protests demanding the dismantling of a nearby phosphate processing plant began on Friday.

Since early September, Tunisia has seen increasing reports of health issues attributed to the factory, whose potentially cancer-causing waste has long generated discontent among Gabes residents.

A local official speaking on condition of anonymity told AFP more than 120 people were admitted to hospitals in Gabes by midday (1100 GMT).

Earlier in the day, radio channel Diwan FM cited an education official who said "dozens of students" had been hospitalised in Gabes.

Local defence deputy chief Ghofrane Touati told AFP "there were cases of suffocation" among those hospitalised, without providing the number of cases.

"Others complained of leg pain, numbness, and loss of mobility," she added.

Tawfik Dhaifallah, a resident, said his little sister was "suffocating because of the fumes" emanating from the industrial zone. "That happens every two or three days."

The processing of phosphate rock into fertiliser emits toxic gases such as sulphur dioxide and ammonia.

The main solid waste product is phosphogypsum, which the plant discharges into the Mediterranean.

It contains radium that decays into radon gas, which is radioactive and can cause cancer.

Slah Ben Hamed, the regional secretary-general of Tunisia's main labour union UGTT, told AFP that "more cases of asphyxiation were recorded today among students at a school in Chatt Essalem", located near the plant.

As the plant was inaugurated in 1972, Ben Hamed warned that "gas leaks are inevitable with such old equipment".

Residents of the city of around 400,000 people have been campaigning for decades against the plant's pollution.

In 2017, the government promised to begin its gradual closure, but authorities earlier this year said they would ramp up production instead.

Local campaign group Stop Pollution has called for "the immediate closure" of the factory units it said were "behind the emissions".

President Kais Saied has dispatched representatives from the environment and industry ministries.