* Talks aim to defuse months of political deadlock
* Negotiations to agree new government and election date
By Tarek Amara
TUNIS, Oct 3 (Reuters) - Tunisia's ruling Islamists and their secular opponents will start three weeks of negotiations on Saturday to allow the government to step down and make way for a caretaker cabinet until elections, a labour union mediating the talks said.
The moderate Islamist Ennahda party agreed at the weekend to a deal under which the government would resign after the talks as a way to end months of political deadlock in the country where the Arab Spring uprisings began.
The powerful UGTT union, which brokered talks between the sides, said in a statement on Thursday that the negotiations would begin on Saturday, to make way for a non-partisan administration and set a date for parliamentary and presidential elections.
"The start of the dialogue is a step to ending this crisis," said Lotfi Zitoun, a senior Ennahda official.
The crisis erupted in July after the killing of an opposition leader by suspected Islamist militants - the second this year. The turmoil has weakened the North African country's economic outlook and raised concerns among its international lenders.
Opposition parties took to the streets to demand the Islamist-led government step down after the killing, accusing Ennahda of being lax on suspected militants.
Since autocrat Zine al-Abidine Ben Ali was toppled in 2011, divisions have widened over the political role of Islam in what has long been one of the Muslim world's most secular nations.
Those differences may still hamper talks where both sides must agree on how to finish writing a new constitution, on when to hold elections, and on other issues such as the composition of a new electoral body to oversee any vote.
Tunisia's path towards democracy has been generally peaceful compared to Egypt, where troops overthrew an Islamist president after mass protests against his rule, and neighbouring Libya, where a weak central government is struggling to assert itself against ex-rebel militias.
In contrast, Ennahda shared power in a coalition with two small secular partners, and has tried to defuse concerns that it could seek to impose a strict Islamist programme impinging on Tunisia's liberal education and women's rights.
But the political turmoil, and opposition concerns that the government has failed to clamp down on hardline Islamist militants, have threatened to undermine a transition seen by many as the most promising among the region's nascent democracies.