* Islamist-led government, opposition due to start talks
* Islamist-secularist rift widens where Arab Spring began (Adds opposition position)
By Anis Milli and Tarek Amara
KEF, Tunisia, Oct 24 (Reuters) - Tunisian security forces fired tear gas on Thursday to disperse hundreds of people trying to storm a local government building during demonstrations against the killing of seven policemen by Islamist militants.
Tensions are rising in Tunisia, where the ruling moderate Islamist Ennahda party and the opposition are struggling to start talks to end a paralysing deadlock since the assassination of two secular opposition leaders earlier this year.
Wednesday's police killings delayed the long-awaited negotiations to resume a transition to democracy, nearly three years after the first Arab Spring uprising toppled Tunisian autocrat Zine al-Abidine Ben Ali.
Clashes with security forces erupted at a government building in Kef in northern Tunisia after funerals for the officers, with enraged residents accusing Ennahda of being too lenient with hardline Islamists. Protesters also attacked two local party offices of Ennahda in Kef and Beja, ransacking one and burning furniture in the street. Demonstrators took to the streets in four other cities to demand the Ennahda government resign, residents said.
"Ennahda killed my son, I will not accept consolation only after the departure of Ennahda...They are destroying our country and kill our children," said the mother of Socrate Charni, one of the seven slain policemen.
Divisions between Islamists and their secular opponents have widened in one of the Muslim world's most secular countries.
Prime Minister Ali Larayedh says Ennahda is ready to resign, but insists on the completion of the country's new constitution, the establishment of an electoral commission and a clear election date before handing over power.
Three weeks of talks had been scheduled from Wednesday to decide on a caretaker government and set a date for elections. But the violence forced a delay of the start of the negotiations until Friday and it was not clear if they would take place even then.
After weeks of wrangling over terms for the talks, and fearing Ennahda will cling to power, opposition leaders threatened to suspend the dialogue unless the Islamist party made a clearer statement of its intent to resign in three weeks.
"We want a clear commitment before we start the negotiations," said Hamma Hammami, the leader of Popular Front opposition alliance.
Two months ago the government declared a local hardline Islamist movement, Ansar al-Sharia, to be a terrorist organisation and began a crackdown that authorities say has led to more than 300 arrests.
Islamist violence is less common in Tunisia than in some other North African countries, where al Qaeda-associated groups have a stronger presence. But militants have grown in influence since the Arab Spring felled relatively secular authoritarian leaders who had suppressed Islamists for decades. (Writing by Patrick Markey; Editing by Mark Heinrich and Barry Moody)
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