TUNIS - Tunisia's Court of Cassation was on Thursday reviewing the jail terms of two young men for posting cartoons of the Prophet Mohammed on the Internet, a case that has angered the secular opposition.

"I am at the court, the hearing is about to begin. After the hearing, I will make known the progress of the case," defence lawyer Ahmed Mselmi told AFP.

Jabeur Mejri and his co-defendant Ghazi Beji, who fled abroad and whose whereabouts are unknown, were sentenced in a closed hearing in March 2012 to seven and half years in jail for "publishing works likely to disturb public order" and "offence to public decency."

The pair were accused of posting pictures of the Prophet "in the nude" on their Facebook pages and of others slandering prominent Tunisian figures.

The verdict, which was upheld at an appeals trial three months later, is now being reviewed in Tunisia's highest court, which can either uphold the ruling or send the case back to an appeals court for retrial.

In a letter written in English and French and published this week by his support group, Mejri said he was a non-believer, criticised the lack of freedom of expression in Tunisia and condemned the conditions under which he is being held.

In the letter, dated February 22, 2013 and entitled "Imprisoned because of Atheism and Secularism," he apologised to Muslims "if there is an insult to their sacred Messenger" but said he didn't trust religion and that "Secularism is my prophet."

"There's no freedom of expression here in Tunisia," added the 28-year-old.

"Seven years and six months is a long period to spend within a dark and gloomy small place.

Officers find pleasure to torture me," he wrote in English, adding: "I am forbidden from medicines to care for my illness

and from other rights."

In its original verdict last March, the court said that representing the Prophet "in a degrading and contemptuous manner showing him in positions with sexual connotations" was an "offence to others whose publication was likely to disturb public order."

The case has stirred up controversy in Tunisia, with secular opposition groups and human rights activists arguing that the defendants were convicted for a "crime of conscience."

Critics of the Islamist party Ennahda, which heads the coalition government, have repeatedly accused it of seeking to Islamise society and of using religion to stifle freedom of expression.

Ennahda sought to include a controversial clause in the country's new constitution "criminalising attacks on religious values." The proposal was eventually dropped, but the party still hopes to get such a clause included in the penal code.

The Islamists say they consider freedom of expression a key achievement of the revolution that toppled former dictator Zine el Abidine Ben Ali, but insist that offending the prophet is different, and likely to destabilise the country.

Hardline Muslims have become increasingly assertive since the revolution, and have been linked to a wave of violence which culminated with an attack on the US embassy last September in protest at a US-made film mocking Islam.

The French mission has been under tight military protection since a French satirical weekly printed cartoons caricaturing the founder of Islam at around the same time.