* New constitution key step to democratic progress
* Islamists, opposition made deal to hold elections
* Tunisia's transition seen as model for region
By Aziz El Yaakoubi
TUNIS, Jan 3 (Reuters) - Three years after the revolution that sparked uprisings across the Arab world, Tunisia's parliament began voting on Friday on a new constitution that will help put its turbulent progress to democracy back on track.
Finishing the charter is a key step before a caretaker government takes office to end a crisis between Islamists and secular parties that threatened Tunisia's transition after the fall of the autocrat Zine al-Abidine Ben Ali.
Its final steps to full democracy have been widely watched as a possible model in a region where Egypt, Libya and Yemen, which also ousted their leaders in 2011, are struggling with violence and instability as well as resurgent Islamism.
About 192 of the National Assembly's 217 members attended the opening debate, where lawmakers began voting one by one on the introductory clauses to the constitution. Approval of all 146 articles is likely to take at least a week.
"This is a constitution for all of Tunisia's people," Mustafa Ben Jaafar, president of the transitional parliament, said at the session. "We are working hard with different parties to finish this process."
The Islamist party Ennahda, which came to power two years ago, and the mostly secular opposition parties have agreed to finish the handover to the caretaker government by Jan. 14, the third anniversary of Ben Ali's fall, having passed the constitution and named an election date and commission.
Initial debate over the drafting of the constitution was hampered by intense divisions over the role of Islam in a country that is one of the most secular in the Arab world. But the parties have overcome their disagreements.
The first clause of the draft constitution says Tunisia is "a free country, independent, with sovereignty; Islam is its religion, Arabic its language and the republic its regime".
Tunisia's transition has been mostly peaceful since its 2011 uprising, but divisions between Islamists and secular opposition parties led to a political deadlock that threatened to unhinge its nascent democracy.
The assassinations of two secular politicians this year and accusations that Ennahda was turning a blind eye to Islamist militancy and religious intolerance threatened to derail progress toward new elections.
But Ennahda and the main opposition party Nidaa Tounes compromised with a deal that calls for the Islamist-led government to step down in favour of a non-political cabinet that will run the country until the elections later this year.
Heavy security was deployed around the parliament building in the capital Tunis on Friday to deter attacks by radical Islamists opposed to the new constitution.
Tunisia said on Thursday it had arrested four Islamist militants and seized arms and explosives in the southern town of Sidi Bouzid, cradle of the revolution against Ben Ali. (Reporting By Aziz El Yaakoubi; Writing by Patrick Markey; Editing by Kevin Liffey)
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