The Interior Ministry said in a statement that the suspects came from around Tunisia and gathered in the area around Menihla, a working class area about 10 kilometers (six miles) from the capital Tunis, and in the locality of Ettadhamoun, which adjoins Menihla.
The ministry did not elaborate on the plans it hopes the police have foiled, nor the identity of those killed and arrested. However, the ministry did say that Kalashnikov rifles, pistols, grenades and ammunition were recovered in the operation.
Tunisia, which is wedged between Algeria and Libya, has been battling extremist violence as it tries to move forward in a fragile economic and political environment.
Two brutal attacks last year, on a luxury beach resort in Sousse and the noted Bardo Museum, together killed some 60 people, mainly foreign tourists. The Islamic State group claimed responsibility for both attacks.
For two weeks in March, security forces battled armed groups, some crossing the border from Libya, who attacked security and military sites in the border town of Ben Guerdane, with the alleged goal of creating an "emirate." Security forces killed 36 attackers and lost 12 agents while seven civilians were killed.