Four people died in clashes between protesters and police in the northwestern Tunisian town of Kef, union activists and witnesses said.
Several hundred demonstrators had been calling for the city's police chief, Khaled Ghazouani, to be sacked for abuse of power, the government news agency TAP reported.
The situation degenerated when Ghazouani slapped one of the protesters and the crowd tried to rush the police station and set it ablaze.
Police opened fire, killing two demonstrators, aged 19 and 36, and seriously wounding three others, said union sources and an interior ministry source.
Two of those injured later died of their wounds, union activists and a local resident told AFP later Saturday.
A union source said Ghazouni had subsequently been arrested and the situation was calm late Saturday.
The official TAP news agency meanwhile reported the arrests of two members of the security forces suspected over the deaths of two detainees in Sidi Bouzid, in the centre of the country.
On Friday, several hundred people had demonstrated in front of the police station there after medical staff at the local hospital said they had found burn marks on the victims' bodies.
In the ensuing unrest they had burned three police cars, a witnessed told AFP.
Confirming the deaths, Interior Minister Farhat Rajhi, said they might have been committed by supporters of the ousted Ben Ali regime.
It was in Sidi Bouzid that a young man, Mohamed Bouazizi, died after setting himself on fire on December 17, triggering the uprising.
In Tunis meanwhile, dozens of members of Tunisia's main trade union rallied calling for a shakeup of its hierarchy.
"Get lost rotten managers!" members of the General Union of Tunisian Workers (UGTT) chanted in front of the union's Tunis headquarters, calling on its secretary general Abdessalem Jrad to step down.
"We ousted (Tunisian president Zine El Abidine) Ben Ali, the time has come to settle accounts with the UGTT's bureaucratic management which flirts with the transitional government and betrays its base," activist Habib Ayadi said.
The UGTT was a key player in the protests that ultimately ousted Ben Ali on January 14.
It briefly joined the transitional government of Prime Minister Mohammed Ghannouchi before its members resigned -- although it has offered qualified backing of the since-reshuffled interim government.
A new splinter union, the General Confederation of Tunisian Workers (CGTT) was announced on Tuesday.
In an effort to get the country back to normal, Tunisia's transitional government announced a two-hour shortening of the curfew, which now begins at midnight and ends at 4:00 am (0300 GMT).
Only a few dozen young people still stage peaceful rallies against the former ruling Constitutional Democratic Rally (RCD) party on the city's central Habib Bourguiba artery, until recently the scene of massive protests.
At one end, a few armored vehicles belonging to the army are parked in front of the interior ministry. But the machine guns once stationed there are absent.