Military Soldiers were deployed in the centre of Tunis on Wednesday after a wave of unrest that officials say has killed 23 people spread to the capital for the first time.
In a provincial town that was the scene of some of the worst clashes at the weekend, witnesses said a large crowd had gathered to demand the resignation of President Zine al-Abidine Ben Ali and that police were nowhere in sight.
People taking part in the weeks of unrest say they are angry about unemployment, corruption and what they say is a repressive government. Officials say the protests have been hijacked by a minority of violent extremists who want to undermine Tunisia.
The protests - which have now been continuing for nearly a month - are the worst in the north African country for decades. They are being watched closely in other countries in the Arab world with the potential for social unrest.
In the strongest U.S. statement on the violence to date, State Department spokesman Mark Toner said Washington was "deeply concerned by reports of the use of excessive force by the government of Tunisia."
On the main avenue in Tunis, two military vehicles were parked opposite the French embassy, and two soldiers with weapons were patrolling in the street, a Reuters reporter said.
A short distance from downtown Tunis, two Humveee vehicles were parked at the entrance to the state television headquarters and two soldiers wearing helmets and flak jackets were patrolling with automatic weapons.
Late on Tuesday, police fired into the air to disperse a crowd ransacking buildings in a Tunis suburb. There were no reports of any casualties from those clashes.
Officials said the civilian deaths - almost all of them in clashes in provincial towns at the weekend - came about when police fired on rioters in legitimate self-defence.
Two witnesses said that several thousand people had come out into the streets in Gassrine, about 200 km (120 miles) from Tunis to protest against the government and the crackdown on the protests.
People were chanting: "Go away Ben Ali," one witness, Mohsen Nasri, told Reuters by telephone.
"There are about 3,000 people here protesting," said a second witness. "There are no police, they have fled to their barracks