TUNIS (AFP) - Tunisia said its forces killed a "terror" suspect in a dawn raid on Sunday and separately foiled a political assassination, like the one that has plunged the country into crisis.

The announcements came after rival protests for and against the Islamist-led coalition government, with the opposition demanding the resignation of the cabinet and the dissolution of the National Constituent Assembly.

The calls have mounted ever since the murder nearly two weeks ago of opposition MP Mohamad Brahmi, the second anti-government politician gunned down in Tunis since February.

Officials have said the same gun was used in both killings and pointed the blame at jihadists but failed to make any arrest in either case.

The interior ministry said the dawn raid targeted a house in the southern Tunis suburb of Ouardia where a group of "key terror" suspects was hiding.

"One of the members of the group was eliminated in an exchange of fire, and four were arrested," the ministry said in a terse statement that did not spell out why the suspects were wanted.

Overnight, the ministry said that police had foiled a new bid to assassinate a political figure and that two "very dangerous terrorists" had been arrested and guns and grenades seized.

The ministry did not identify the target of the plot but said it had taken place in Sousse, 140km south of Tunis, on Friday.

Police traded fire with gunmen after searching a house in the area, and a third suspect fled, it added.

The security force operations came as Tunisia's moderate Islamist Ennahda party was battling to defend the position at the head of the governing coalition it won in an October 2011 election.

Government critics have accused Ennahda of failing to rein in rampant violence blamed on radical Islamists since the 2011 uprising that ousted veteran president Zine El Abidine Ben Ali.