Tunisia's public prosecutor on Wednesday ordered the detention of presidential candidate Ayachi Zammel pending trial, the latest move in what critics call manoeuvres by President Kais Saied to remove rivals challenging his re-election bid.

Zammel was arrested on Monday on suspicion of falsifying popular endorsements. He was one of the three candidates approved to run in the election expected on October 6, along with Saied and politician Zouhair Maghzaoui.

Zammel has strongly denied the accusations and said he was facing "restrictions and intimidation" because he was a serious competitor to Saied.

Zammel's lawyer Kais Oueslati told Reuters that the prosecutor had decided to keep him in custody pending trial, the date of which has not yet been set.

The decision came two days after Tunisia's electoral commission defied an administrative court ruling to reinstate three prominent candidates in the race.

Rights groups, political parties and constitutional law professors protested, saying the decision was an unprecedented step that raised doubt about the legitimacy and legality of the election in the North African country.

Saied, a retired law professor, was democratically elected in 2019, then tightened his grip on all powers in 2021 in a move the opposition described as a coup.

Saied has denied carrying out a coup and said his steps were legal and meant to end years of chaos and corruption. He said last year he would not hand over Tunisia to "non-patriots".

The election campaign is set to formally begin on September 14 amid calls by critics of Saied on all his election rivals to withdraw from the race, calling the vote a farce.

They said the electoral commission was no longer independent and its sole goal had become ensuring Saied's return for a second term. The electoral commission denies such allegations, saying it is just applying the law and is neutral.