Police officers are hard pressed to restore order in the capital Tunis and other cities [AFP]

A security vacuum left by the departure of Zine El Abidine Ben Ali, the Tunisian president, is being exploited by looters and violent gangs, witnesses say.

Residents in several parts of the Tunisian capital, Tunis, said on Saturday that groups were prowling through neighbourhoods setting fire to buildings and attacking people and property, with no police in sight.

Occasional gunshots could be heard in the centre of Tunis as well as the sound of tear-gas grenades being fired, while helicopters patrolled overhead and acrid smoke hung in the air, Reuters news agency reported.

Several witnesses in Denden, 19km from Tunis, said soldiers were dropped by helicopter to try to restore security.

Witnesses told Al Jazeera that masked special forces they suspected of being affiliated to the toppled government, or foreign militias imported by the leadership before exiting the country, were cracking down on looters.

They said that the army published help phone lines for citizens to call to report pillaging and security emergencies.

Martial law

In a dramatic climax to weeks of violent protests against his rule, Ben Ali, who ruled Tunisia for more than 23 years, was pushed out on Friday and Mohamed Ghannouchi, the prime minister, took over as caretaker president.

In an interview with Al Jazeera, Ghannouchi said that everything was being done to restore order.

"Gangs are indulging in looting, wreaking havoc and destruction and spreading fear among citizens. We call on Arab states to help pacify the situation in Tunisia," he said.

"Police are patrolling the streets to restore security and protect public property. Martial law is in effect and the army is deployed in critical and strategic areas.

"We aim now to get things back to normal and restore security."

In working-class suburbs of Tunis, hundreds of residents lined the streets with metal bars and knives trying to ward off looters.

Sources told Al Jazera that there were calls in Tunisia to form civil squads to defend quarters, combat looting and take control of security because the army was only stationed in certain areas.

"There is a terrible state of fear. May God bring us peace," one woman, Lilia Sfaxi, told Reuters. "We cannot live any more like this in total fear."