Chaotic scenes marred Tunisia's first parliamentary debate on a new post-Arab Spring constitution on Monday, as tensions flared between the ruling Islamists and their opponents over the long-delayed draft.

The North African country's political stability remains fragile, two and a half years after the revolution that ousted veteran strongman Zine El Abidine Ben Ali, adding urgency to the need for the new charter.

But the debate was suspended after less than 30 minutes when the presentation by the head of the drafting committee, Islamist MP Habib Kheder, was interrupted by opposition MPs who accuse him of introducing controversial articles in a discretionary manner.

Those articles exempt laws adopted under ruling Islamist party Ennahda from constitutional control for three years, and extend indefinitely the legislative powers of the assembly while failing to draw up a timetable for elections to replace it with a new parliament.

When Khoder finally returned to the podium to resume his presentation at around 6 pm (1700 GMT), he was again drowned out by heckling from opposition MPs who shouted slogans and sang the national anthem.

Before the session was interrupted, a handful of secular opposition MPs issued a statement criticising the "fraudulent process that has affected the workings of the constituent committees."

The assembly's deputy speaker, Islamist MP Meherzia Laabidi, strongly criticised the disruption to the debate.

"The men of Tunisia remained seated... These were dwarfs who stood up, she told private radio Mosaique FM, condemning the "immaturity in the ranks of the opposition."

Several hundred people also protested against the draft constitution outside the National Assembly, which has been repeatedly criticised for its inefficiency and the non-attendance of members.

Beyond the controversial clauses introduced, some civil society and opposition activists have raised concerns that the text does not sufficiently guarantee basic liberties.

Amnesty International said it undermined the principles of international human rights law by giving greater legal value to the constitution than to international treaties ratified by Tunis.

But a relatively wide consensus appears to have been forged on the division of executive powers between the president and the prime minister, which was at the centre of a major political tussle.

Ennahda had previously insisted on a pure parliamentary system before allowing the head of state to retain important powers, notably in the areas of defence and diplomacy. The Islamists also agreed to abandon having the Koran inscribed as a source of law.

Assembly speaker Mustapha Ben Jaafar, who heads Ennahda's secular ally Ettakatol, sought to highlight the strengths of the draft text.

"We wanted to build a constitution on solid bases, guaranteeing freedoms, human rights and the rights of women," adding that it did "not represent one specific party... but is a constitution for all Tunisians".

The text must be approved by a two-thirds majority in parliament or be put to a referendum.

The main political parties were originally given a year to draft the text after the first post-revolution elections in October 2011, but the timetable was repeatedly revised amid disagreement about the nature of Tunisia's future political system.

The new constitution must be adopted before a timetable can be drawn up for fresh elections which Prime Minister Ali Larayedh has promised to hold before the end of 2013.

Adopting the new constitution is seen as key to restoring stability in Tunisia and helping overcome the political crises, social unrest and violent attacks by radical Islamist groups that have rocked the country since the revolution.

"We must work together to put an end to this transitional phase by organising elections as soon as possible," Ben Jaafar told parliament on Monday.

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