Tunisian President Moncef Marzouki on Friday asked Interior Minister Ali Larayedh, a hardliner from the main Islamist Ennahda party, to form a government within two weeks, his spokesperson said.
Marzouki's spokesman told a news conference Ennahda leader Rached Ghannouchi had formally nominated Larayedh to succeed outgoing Prime Minister Hamadi Jebali, who resigned on Tuesday.
Tunisia plunged into political crisis on 6 February when the assassination of secular opposition politician Chokri Belaid on 6 February ignited the biggest street protests since the overthrow of strongman Zine al-Abidine Ben Ali two years ago.
Ennahda's choice for prime minister raised hackles among opposition parties, some of whom accuse Larayedh's Interior Ministry of failing to curb Islamist violence, although he is credited for acting firmly against Al-Qaeda-linked militants.
"The decision deepens the crisis because Larayedh headed the ministry responsible for the killing of Belaid and violence that has spread throughout the country," said Zied Lakhdar, a leader in the Popular Front, in which Belaid was secretary-general.
The Interior Ministry and Ennahda have denied they had any hand in Belaid's killing, which they have condemned.
Mahmoud Baroudi, a leader of the secular Democratic Alliance opposition party, said Larayedh's appointment would aggravate tensions and increase anger in the streets.
"He was responsible for leniency with Islamist violence against human rights activists," he said, blaming Islamists for disrupting opposition meetings and assassinating Belaid.
No one has claimed responsibility for Belaid's killing.
Jailed under Ben Ali
Ennahda is the biggest party in the National Constituent Assembly with 89 of its 217 seats. Marzouki's secular Congress for the Republic party (CPR), the second largest with 29 seats, has already said it will join a new Ennahda-led coalition.
Larayedh, 57, is viewed as part of Ennahda's hardline wing, which rejects any role for parties linked to the Ben Ali era.
A maritime engineer, Larayedh spent 15 years in jail under Ben Ali. He became interior minister when Jebali's government was formed in December 2011 after an election in October.
Jebali, who remains Ennahda's secretary-general, refused to head the next government after his own party rejected his plan for an apolitical technocrat cabinet to prepare for elections.
He was seen as a moderate overruled by Ghannouchi, who says the last election gave Ennahda a popular mandate to rule in a power-sharing deal with moderate secular parties.
"Larayedh is not a man of consensus," said Nejib Chebbi, leader of the secular Republican Party. "He failed during his work as head of the Interior Ministry."
Tunisia, whose transition from autocracy has so far been far less violent than those in neighboring Libya and Egypt, faces rifts between Islamists and their opponents that threaten the stability of its fledgling democracy and ailing economy.
Political uncertainty has put negotiations on a US$1.78 billion loan from the International Monetary Fund on hold and prompted Standard and Poor's to lower its long-term foreign and local currency sovereign credit rating on Tunisia on Tuesday.
While Ennahda and Marzouki's CPR together would wield a narrow majority in the assembly, Larayedh may seek additional partners. The leftist Ettakatol party, which joined Jebali's government, has not said if it will serve under Larayedh.
Hechmi Hamdi, leader of the moderate Islamist Popular Petition party, which holds seven assembly seats, said his group would stay in opposition, accusing Larayedh of responsibility for the death of two prisoners on hunger strike and the torture and ill-treatment of protesters in the restive Siliana region.