Tunisian police on Monday detained Ahmed Souab, a prominent lawyer and fierce critic of the country's president, lawyers told Reuters, raising human rights groups' concerns that a crackdown on dissent will go ahead.

Dozens of activists took the streets in protest, raising slogans against President Kais Saied and demanding an end to the harassment, silencing and imprisonment of critics.

Souab is among the lawyers acting for opposition leaders who received lengthy prison sentences on Saturday on conspiracy charges.

Souab strongly criticised the judge and the trial on Friday, calling it a farce and saying the judiciary had been completely destroyed.

Protesters on Monday marched to Habib Bourguiba Avenue, the main thoroughfare in Tunis, and chanted, "No fear, no terror, the street belongs to the people." "Dictator Saied, your turn has come".

"It seems he was detained because of his critical comments on the trial on Friday," said Samir Dilou, one of Souab's lawyers. Two others lawyers confirmed the detention.

A spokeswoman for the terrorism court said Souab was detained in a case on "terrorism-related charges" over a video which was seen as a threat to judges.

Political parties rejected the rulings, saying they were retaliatory after a trial aimed at cementing Saied's authoritarian rule.

Rights groups say Saied has had full control over the judiciary since he dissolved parliament in 2021 and began ruling by decree. He dissolved the independent Supreme Judicial Council and sacked dozens of judges in 2022.

"The mass conviction of dissidents...is a disturbing indication of the authorities' willingness to go ahead with its crackdown on peaceful dissent," the human rights group Amnesty International said.

Those convicted included prominent leaders of the Islamist Ennahda party, the main opposition party to Saied.

Ennahda's vice president, Noureddine Bhiri, received a 43-year prison sentence, while the court sentenced two senior party officials, Said Ferjani and Sahbi Atig, to ​​13 years each.

The largest sentence was 66 years for businessman Kamel Ltaif, while opposition politician Khyam Turki received a 48-year sentence.

Authorities say the defendants, who include former head of intelligence Kamel Guizani and the former head of Saied's office, Nadia Akacha, tried to destabilise the country and overthrow Saied.

The opposition leaders denied the accusations and said they were preparing an initiative aimed at uniting the fragmented opposition.

Saied said in 2023 that the politicians were "traitors and terrorists" and judges who would acquit them were their accomplices.