A Tunisian court has sentenced opposition leader Rached Ghannouchi to 20 years in jail for "plotting against the state", his lawyer told AFP on Tuesday, increasing the sentence by six years.

The head of Tunisia's Islamist movement Ennahdha Rached Ghannouchi greets supporters upon arrival to a police station in Tunis ,on February 21, 2023, in compliance to the summons of an investigating judge. (Photo: AFP)

The charge is one of several for which Ghannouchi has already been in prison since 2023, with the 84-year-old now facing a total of more than 40 years behind bars.

Twenty-two people stood trial alongside Ghannouchi last year accused of offences that include "plotting against state security", which Ghannouchi's defence team called "fabricated" and politically motivated.

An appeals court has upped the sentence from 14 years to 20, Ghannouchi's lawyer Samir Dilou told AFP on Tuesday.

Ghannouchi leads Tunisia's Islamist-inspired Ennahdha party and has been a staunch critic of President Kais Saied.

His defence team said he would not appeal against the latest ruling, citing a lack of guarantees for a fair trial.

Tunisia has emerged as the Arab world's only democracy following the ouster of longtime ruler Zine El Abidine Ben Ali in 2011, which kicked off the Arab Spring uprisings.

But since Saied came to power in 2019, local and international NGOs say there has been a regression of rights and freedoms.

Ennahdha played a key role in Tunisian politics for years before Ghannouchi was hit with multiple prison terms, which include a 22-year sentence on a separate charge of plotting against state security.

Other defendants in the latest trial include former presidential chief of staff Nadia Akacha and former prime minister Youssef Chahed, who are both currently abroad.

They faced charges including "conspiracy against the state's internal security" and "forming an organisation and an agreement linked to terrorist crimes", according to court documents.

Ghannouchi and other senior Ennahdha figures, including his son-in-law Rafik Abdessalem and retired military officer Kamel Bedoui, were also accused of setting up a "secret security apparatus" serving the party.

Their trial followed a similar one last year in which some 40 opposition figures were sentenced to up to 45 years in prison.

Most of Saied's opponents -- whether from the left or right -- have either been detained or have fled the North African country.