* Tensions rise over threats to secular lawmakers
* Ruling Islamists Ennahda have agreed to step down
* Three-year anniversary of uprising approaching
Death threats against Tunisian secular lawmakers on Sunday disrupted voting on a new constitution, underscoring tensions over the role of Islam and the transition to democracy three years after the nation's revolution.
Tunisia's parliament started voting last week on the new charter, which is meant to put democracy back on track after deadlock between ruling Islamists and secular parties since the 2011 fall of autocratic leader Zine al-Abidine Ben Ali.
Approving the constitution is a key step before a caretaker government takes office to end the crisis between Islamists and secular opponents and prepare for new elections later this year.
Members of the national assembly have approved several articles of the new constitution, but on Sunday, Mongi Rahoui of a leftist opposition party and two other secular opposition members received anonymous death threats, officials said.
Debate in parliament was dominated on Sunday by discussion of the threats, leading to a suspension of planned debate and voting on the constitution.
Since the 2011 uprising, tensions over the role of Islam in Tunisia and the assassination of two secular politicians by hardline Islamist militants last year have widened divisions between Islamists and secular parties.
"According to information received, assembly member Mongi Rahoui and two others may have been threatened with aggression and the ministry has taken steps to protect those individuals and opened an investigation," the Interior Ministry said.
Rym Mahjoub, an opposition member of the assembly, told Reuters that police came early Sunday morning to Rahoui's home to warn him that a militant threat had been issued against him, without giving further details.
Opposition party members said the threats came after Habib Ellouz, a hardliner from the ruling Islamist Ennahda party, told reporters on Saturday that Rahoui was seen as an enemy of Islam who wanted to remove references to Islam from the constitution.
Ennahda lawmakers said Ellouz, who apologised on Sunday, had only expressed his personal views, which did not reflect the party's position.
Drafting Tunisia's new constitution began two years ago, but its approval has been delayed by disagreements over the role of Islam in politics, and by widening divisions between Ennahda and its secular opponents.
Tunisia's final steps to democracy have been seen as a model for other countries in the region since the 2011 Arab Spring revolts toppled leaders in Egypt, Libya and Yemen, and triggered the war in Syria.
The Islamist party Ennahda has agreed to step down under a deal that will see parties finish the constitution, set a date for elections and name an electoral council before a non-political caretaker government takes over.
Controversy over the role of the religion in politics has resurfaced in the voting with many constitutional articles judged as too vague by secular parties. But both sides have agreed over the main philosophy of the draft.
The draft constitution recognizes Islam as the religion of the state, but also guarantees religious freedom for Tunisians.
One of the most secular nations in the Arab world, Tunisia has seen ultra-conservative Islamists rise since its revolution. Long oppressed by Ben Ali, many hardline Islamists returned from exile or been released from jail since 2011.