Thousands of protesters have assembled in the Tunisian capital of Tunis after the assassination of a leading opposition politician.
Mohamed Brahmi was shot dead in the second such political killing this year.
The Tunisian interior minster said Mr Brahmi was shot using the same gun that was used to kill his party leader Chokri Belaid in February.
Lotfi Ben Jeddou told a news conference: "The same 9mm automatic weapon that killed Mr Belaid also killed Mr Brahmi."
He named the main suspect as hardline Salafist Islamist Boubacar Hakim, already being sought on suspicion of smuggling weapons from Libya.
The killing has prompted violent protests against the Islamist-led government across the country and a strike call by the main trade union body, the UGTT.
Protesters assembled outside the UGTT headquarters in central Tunis, preparing to march down the capital's main boulevard as riot police were deployed.
In a reference to the country's Islamist-led government, the demonstrators chanted: "Down with the rule of the Muslim Brotherhood."
The protest followed calls by the secular opposition for street rallies to topple the government led by the Islamist Ennahda Party.
Mr Brahmi belonged to the secular Popular Front party, whose then-leader, Mr Belaid, was also shot dead on 6 February.
Mr Belaid's killing set off the worst violence in Tunisia since President Zine al-Abidine Ben Ali fell in 2011 in the first of the Arab Spring revolutions.
Mr Brahmi was a critic of the Ennahda-led ruling coalition and a member of the Constituent Assembly that has drafted a new constitution for the North African nation.
Authorities said Mr Brahmi had been shot 14 times.
German Chancellor Angela Merkel condemned what she called the "cowardly assassination" of Mr Brahmi and demanded that his and Mr Belaid's killers be swiftly brought to justice.
Shops and banks were shut in anticipation of violence, while all flights in and out of the country were cancelled because of a strike.
Divisions between Islamists and their secular opponents have deepened since the popular uprising against Mr Ben Ali.
Mr Brahmi's family said his funeral would take place on Saturday and he would be buried near the tomb of Mr Belaid.
Secularist calls for the dissolution of the government and the formation of a national salvation administration were rejected by Prime Minister Ali Larayedh.
Riot police in Tunis fired tear gas in front of the interior ministry to disperse protesters yesterday evening.
Similar demonstrations erupted in the cities of Sfax and Kef and in the southern town of Sidi Bouzid, the cradle of the Tunisian revolution, where protesters set fire to two Ennahda party offices.
Tunisia announced a day of national mourning for today and radio stations broadcast patriotic songs.
Tunisia's political transition since the revolt that toppled Mr Ben Ali has been relatively peaceful, with the moderate Islamist Ennahda party sharing power with smaller secular parties.
But the government has struggled to revive the economy and has come under fire from secularists who accuse it of failing to curb the activities of Salafi Islamists.
The Egyptian army's overthrow of Islamist President Mohammed Mursi on 3 July following mass protests against him has further energised the anti-Islamist opposition in Tunisia.
Rached Ghannouchi, the Ennahda party leader, said the attack on Mr Brahmi was aimed at "halting Tunisia's democratic process and killing the only successful model in the region, especially after the violence in Egypt, Syria and Libya".
"Tunisia will not follow the Egyptian scenario, we will hold on."
The assassination occurred on Tunisia's Republic Day, as the country prepares to vote in the next few weeks on the new constitution before a presidential election later in the year.
The turmoil dealt another blow to efforts to revive Tunisia's vital tourism industry. Cultural events, including the Carthage Festival, were suspended following Mr Brahmi's killing.