* Sunday's election is first in wake of Arab Spring uprisings
* Islamist party likely to win biggest share of vote
* Secularists say their liberal values are under threat
* Ordinary Tunisians preoccupied with economic woes
By Christian Lowe and Tarek Amara
TUNIS, Oct 21 (Reuters) - Tunisians will hand a share of power to an Islamist party when they vote on Sunday in an historic first democratic election which could set the template for other Arab countries convulsed by the "Arab Spring" uprisings.
The birthplace of the revolts which re-shaped the political landscape of the Middle East, Tunisia in January forced autocratic leader Zine al-Abidine Ben Ali to flee to Saudi Arabia and set in train a transition to democracy.
The new freedoms have allowed the moderate Islamist Ennahda party -- banned by Ben Ali -- to emerge as front-runner in the vote. That though is causing anxiety among the country's secular elite, which believes its liberal values are under threat.
With so much at stake, there are concerns that even the smallest question mark over the legitimacy of the vote could bring supporters of rival parties out onto the streets.
The government says 40,000 police and soldiers will be deployed to prevent any protests spilling over into violence. Shopkeepers say people have been stockpiling milk and bottled water in case unrest disrupts supplies.
"The elections are a big party which will be celebrated by everyone, whatever the results," said Kamel Jendoubi, a human rights campaigner exiled by Ben Ali for 17 years who is now in charge of running Sunday's vote.
But a failure of the election process "would be catastrophic for North Africa and the Middle East region", he said. "Tunisians must take this important responsibility to light the path of democracy for the peoples of the region."
Ten months ago, Ennahda's leader Rachid Ghannouchi was in exile in London and hundreds of his followers were in jail. Since then, his party has tapped into a desire among many Tunisians for leaders who reflect their Islamic faith after years of Ben Ali's aggressively enforced secularism.
"I've chosen to vote for Ennahda because they are closest to Islam," said 56-year-old Mokhtar Bahrini, a retired civil servant. "They're very moderate and they're not radical ... We have to give them their chance."
The party showcased its tolerance by naming as one of its candidates a woman who spurns the hijab, or Islamic headscarf, in favour of dark glasses and a trendy white sports cap.
But secularists in Tunisia -- whose first post-independence president called the hijab an "odious rag" and where religion has always been kept out of politics -- do not trust Ennahda's assurances.
Some predict the erosion of women's rights and a clamp down on bars and cinemas, though Ennahda denies any such plans. Violent protests last week over the broadcast of a film that Islamists considered blasphemous deepened secularists' worries.
"Personally, I don't favour a party that speaks in the name of Islam," said Samir Ayed, a 30-year-old taxi driver. "I want a party of the left that will safeguard the liberties we acquired after so many years of repression."
In Egypt, whose revolution was inspired by the one in Tunisia, the military act as guarantor of political stability. But in Tunisia, the army had kept out of politics.
Tunisians are voting for a special assembly which will draft a new constitution, choose new interim executive bodies and schedule a fresh round of elections.
Opinion polls suggest Ennahda, the Arabic word for Renaissance, will win between 15 and 25 percent of the vote, with its nearest rivals lagging well behind.
"I think it is safe to say that Ennahda will be the largest party short of a majority," said a Western diplomat, speaking on condition of anonymity.
The election is likely to be followed by days or weeks of horse-trading. Ennahda will try to cut deals with other parties to assemble a majority while its secularist rivals will try to sideline it by building alliances of their own.
As the probable biggest party, Ennahda is favourite to form a majority coalition. To do that it may need to bring on board some of the secular parties, diluting the Islamist influence.
"The lack of a clear electoral mandate is likely to lead to moderation rather than radical change," said Crispin Hawes, Eurasia Group's director for Middle East and Africa.
If Ennahda does come to power, it will be the first election win for Islamists in the Arab world since Hamas won a 2006 vote in the Palestinian territories. Before that, Islamists won a 1991 legislative election in Algeria but that was annulled by the military and a brutal conflict followed.
Egypt in particular will be watching Tunisia's vote keenly. That country will start voting for a post-revolution parliament next month and the Muslim Brotherhood -- close in ideology to Ennahda -- is likely to do well.
Tunisians take a quiet pride in the fact they are holding their first free election and blazing a trail for the Arab world, but there is no jubilation on the tree-lined streets of this former French colony.
The problems that were at the root of the revolution, unemployment and poverty, are still there. If anything, they have grown worse because large numbers of tourists, a huge source of revenue, were scared away by the revolution.
Whoever takes power after Sunday's election will need to address those issues if Tunisia's revolution is to succeed.
"Why should a population support a democracy if they do not get anything, if the work does not come?" said the Western diplomat. (Additional reporting by Andrew Hammond; Editing by)