Tunisian police have tear-gassed hundreds of protesters who tried to storm a local government building.

Demonstrations had broken out over the killing of seven police officers by Islamist militants on Wednesday, with a general strike being held in two cities.

Clashes erupted at a government building in Kef in the north of the country on Thursday after funerals for the officers, with enraged residents accusing the ruling Islamist Ennahda party of being too lenient with Islamist extremists.

Protesters attacked two local Ennahda party offices in Kef and Beja, ransacking one and burning furniture in the street.

Unusually, the National Guard Union, which represents the police, took to the streets and was joined by thousands of students, who marched on the two offices and burned them to the ground as police stood by.

Demonstrators also marched in four other cities to call on the government to resign immediately.

Ennahda and the opposition have been preparing to start talks to end a paralysing deadlock since the assassination of two secular opposition leaders earlier this year.

But Wednesday's killings delayed the long-awaited negotiations and the National Salvation Front opposition said Ennahda had until this morning to resign or face broader protests.

It called on supporters to step up a campaign of civil disobedience.

The general strike was called by the powerful UGTT trade union confederation for two regions of the country, including Sidi Bouzid, where Wednesday's fatal clash broke out.

Prime Minister Ali Larayedh gave on Thursday what the opposition described as an "ambiguous" commitment to step down.

"We repeat today our commitment to the principle of relinquishing power in line with the different phases envisaged in the road map," he said.

But that was not good enough for the Popular Front, a group of left-wing opposition parties headed by ex-premier Beji Caid Essebsi, who has been an outspoken critic of the Islamists.

UGTT chief Houcine Abassi said that the premier needed to clarify his comments to get the national dialogue back on track.