Three years after its uprising, Tunisia's parliament began voting Friday on a long-delayed new constitution whose adoption would mark a crucial democratic milestone in the birthplace of the Arab Spring.

A tight deadline of January 14 has been set for the adoption of the charter, which could end months of political crisis and further distance Tunisia from the chronic instability plaguing other countries in the region rocked by regime change.

On that day, Tunisians will celebrate the anniversary of the 2011 overthrow of former strongman Zine El Abidine Ben Ali.

At Friday's opening session, lawmakers approved the title of the charter, by 175 votes out of the 184 MPs present, and the first three paragraphs of the preamble.

These refer to "the pride of our people in the struggle to gain independence... and then to get rid of tyranny," as well as "the attachment of our people to the teachings of Islam".

Before the vote, speaker Mustapha Ben Jaafara said Tunisia had been through "difficult moments, marked by a lack of trust," and that adopting the charter was "a complicated step which requires sacrifices and patience".

Elected in October 2011, the National Constituent Assembly was due to have drafted and adopted the text within one year.

But the process was heavily delayed by deep divisions between the ruling Islamist party Ennahda and the opposition, aggravated by a rise in attacks by Islamist militants and sometimes violent social unrest.

The deadlock, which became a full-blown crisis with the assassination in July of an opposition MP by suspected jihadists, paralysed political life and prevented the formation of functioning state institutions.

Ennahda and the opposition negotiated a series of compromises during intense negotiations in recent weeks, aimed at securing the approval of two thirds of the assembly's 217 elected members needed for the constitution to be adopted.

In the absence of such a majority, it must be put to a referendum.

But if approved by this month's deadline which Tunisian main parties have committed to, the charter is expected to end the impasse.

High hopes

The presidency expressed confidence that lawmakers would "do everything to ratify the constitution before January 14, so that that day marks the fall of dictatorship and the advent of democracy."

Adopting the new constitution, as well as an electoral law and commission, should lead to the departure of the outgoing Islamist-led government and the appointment of technocrat premier Mehdi Jomaa, nominated in December under a deal to end the crisis.

The powerful UGTT trade union, the main mediator between the Islamists and the opposition, repeated the deadline for forming the new government of independents was next Wednesday.

After examining the charter's preamble, MPs are to scrutinise the 146 articles finalised in June and some 30 key amendments drafted during the recent negotiations.

Another 200 amendments have been proposed, including an attempt to make Islamic sharia law a main source of legislation, but they are thought to have little chance of succeeding.

During the negotiations, the parties agreed to keep the main article of independent Tunisia's first constitution, which gives Islam a vague status, after Ennahda renounced its demand that sharia be enshrined in the text.

"Tunisia is a free, independent and sovereign state. Islam is its religion, Arabic is its language and it is a republic," the article says.

Another key compromise concerns the powers of the head of state, in a country that recently emerged from five decades of dictatorship.

The Islamists, who were persecuted under Ben Ali and had argued for maximum limitations, finally agreed to divide power between the president and the government.

On human rights, the draft text guarantees freedom of expression and conscience, freedom of assembly and the right to strike.

"The constitution will be one of freedom, of independence and of justice," Ben Jaafar promised on Thursday, amid high hopes among politicians that the charter represents a key step towards Tunisia becoming the Arab world's first true democracy.

But rights organisations including Amnesty International and Human Rights Watch jointly warned some articles were too vague, and called for a provision stating the principle of equality between men and women.