Thousands have rallied in the southern coastal city in recent days calling for the closure of a phosphate processing plant, which they say is behind a rise in gas poisonings and other pollution-related health problems.

"The arrests targeted night protesters," said Mehdi Talmoudi, a lawyer and member of the local branch of the Tunisian Bar Association.

"While daytime demonstrations have been largely peaceful, those at night have seen occasional clashes with security forces and burning tyres," he told AFP.

Talmoudi said the exact number of arrests was not known.

But Khayreddine Debaya, coordinator of the local campaign group Stop Pollution, said "over 100 people were taken into custody" by early Saturday.

"Police arrested more than 70 people just last night, and more by dawn," he said. "Some were taken from their homes."

Other Tunisian activists on social media have also condemned "a wave of arrests".

Locals in Gabes have held several rallies urging the closure of the factory, which processes phosphate to make fertilisers.

They say it has recently been releasing more toxic gases and radioactive waste into the sea.

Authorities earlier this year said they would ramp up production at the plant, despite a 2017 promise to gradually shut it down.

Early on Saturday, the Tunisian presidency said President Kais Saied had summoned parliament speaker Brahim Bouderbala and the head of the second parliamentary chamber, Imed Derbali, to discuss "the environmental situation" in Gabes, among other issues.

Saied said "work was underway to find urgent solutions to pollution".

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

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