TUNIS (AFP) - Tunisia's Islamist Prime Minister Ali Larayedh, who was tortured under the ousted Ben Ali regime, resigned on Thursday as part of a plan to end months of political deadlock.

His resignation sees the departure of Tunisia's first democratically elected government, which came to power after veteran strongman Zine El Abidine Ben Ali was overthrown in the first of the Arab Spring uprisings three years ago next week.

"As I promised to a short while ago ... I have just submitted the government's resignation," Mr Larayedh told a press conference.

His announcement comes as part of a blueprint, drawn up by mediators, to put the democratic transition back on track after the assassination of opposition MP Mohamed Brahmi by suspected Islamist militants last year.

Under the plan, he is to be replaced within 15 days by premier designate Mehdi Jomaa at the head of a government of technocrats that will lead the country to fresh elections this year under a new constitution.

"We took on our responsibilities in very difficult conditions," Mr Larayedh said in a speech on state television shortly before handing his resignation to President Moncef Marzouki. "We have worked for the benefit of our country and we respect our commitments."

Mr Larayedh's Islamist Ennahda party had been under mounting pressure to relinquish the grip on power it won after the uprising in elections to a constituent assembly, as the economy has stagnated and social unrest intensified.

Events in fellow Arab Spring country Egypt, where elected Islamist president Mohamed Mursi was overthrown by the army last July after a single year in power, has added to the pressure.

The formation late Wednesday of an independent authority to oversee fresh elections, which the Ennahda party had set as a condition for stepping down, removed the last hurdle to Mr Larayedh's resignation, according to the powerful UGTT trade union confederation, the main mediator in the crisis.

The approval of a new constitution, which Ennahda had also demanded in return for handing over power, is on track to meet an agreed deadline of January 14, the uprising's third anniversary, with the assembly voting on it intensively article by article.

The new charter had been delayed for months by the withdrawal of opposition assembly members in protest at Brahmi's killing in July.

But their return has seen compromises swiftly reached on many of the most divisive provisions, including gender equality and the role of Islam.

On Thursday, the constituent assembly agreed to an article setting a goal of 50-50 representation between the sexes in all elected bodies, an exceptional move for the Arab world but one in keeping with the secularism that Tunisia adopted at independence which has given its women by far the most extensive rights in the region.

The charter needs the approval of two-thirds of the assembly's 217 elected members, and it will then be the job of the next parliament apply it in Tunisia's legislation.