Tunisia's ruling Islamist party office in the town of Kef was torched by angry protesters on Thursday, an AFP photographer reported, amid a main opposition party ruling out talks without the government promising to step down.
The walls of the building were burned and equipment inside it destroyed, with witnesses saying that protesters had ransacked the office in the morning.
The remains of charred documents and tires were strewn in the road outside the building, occupied on the second floor by the ruling Ennahda movement.
The office is located just meters from the family home of one of six policemen killed on Wednesday in a firefight with suspected jihadists.
Meanwhile, one of Tunisia's main opposition parties said that they would not take part in negotiations to end months of political deadlock without a written pledge from the government to quit power.
"Nidaa Tounes believes it is not feasible for the dialogue to begin...without a formal and written commitment by the government to resign," said the party headed by ex-premier Beji Caid Essebsi, who is an outspoken critic of ruling Islamist party Ennahda.
Making way
The opposition demands that the country's Ennahda-led government commit to resign from power within three weeks of the start of talks, in order to make way for a interim cabinet of independents.
Negotiations were due to start on Wednesday, facing fresh delays when critics of Islamist Prime Minister Ali Larayedh rejected his stated commitment to the "principle" of stepping down as "ambiguous."
The long-awaited discussions are the main focus of a roadmap to end the political deadlock gripping the country since opposition MP Mohamed Brahmi was assassinated in July, are now due to start on Friday at 0900 GMT, say mediators.
In addition to as creating a government of technocrats within three weeks, negotiators must take up a new constitution and electoral law, as well a timetable for fresh elections within a month.
Tunisia's democratic transition has effectively been choked by disputes between the Islamist-led coalition and the opposition, nearly three years after President Zine el-Abidine Ben Ali was overthrown.
Since Ben Ali's ouster, country has been destabilized by violence from jihadists, which the opposition accuses the ruling Ennahda party of having failed to subdue.