Tunisia faces a general strike after gunmen shot dead a leading opposition figure in a killing that brought thousands of protesters on to the streets and sparked international condemnation.
Tunisia's national airline Tunisair cancelled all flights on Friday.
MP Mohamed Brahmi, a father of five, was shot by unknown gunmen outside his home on Thursday in the second such political assassination this year.
The ruling Ennahda party, a moderate Islamist group, denied accusations from his family that it was involved.
Protesters took to the streets on Thursday in central Tunis and in Sidi Bouzid, the birthplace of the Arab Spring and Brahmi's home town.
Police in Tunis fired tear gas to disperse scores of demonstrators who tried to set up a tent for a sit-in calling for the fall of the regime.
The General Union of Tunisian Labour (UGTT) called Friday's general strike across the country in protest at "terrorism, violence and murders".
It last called a two-hour general strike on January 14, 2011, the day former Tunisian dictator Zine El Abidine Ben Ali fell.
Brahmi, 58, of the leftist Popular Movement, was killed outside his home in Ariana, near Tunis, Watanya state television and the official TAP news agency reported.
"He was riddled with bullets in front of his wife and children," Mohsen Nabti, a fellow member of the small movement, said in a tearful account aired on Tunisian radio.
Human Rights Watch said that Brahmi's son, Adnen, had told its researchers he heard a first and a second gunshot, then several other shots as if from a machine gun.
He and his sister ran out of the house and as they reached their father's car they saw two men riding off on a motorbike, HRW said in a statement.
The Tunisian presidency, meanwhile, told AFP that Friday would be observed as a day of national mourning "following the assassination of lawmaker martyr Mohamed Brahmi".
The February 6 assassination of Chokri Belaid, another opposition figure, also outside his home, sparked a political crisis in Tunisia and charges of government connivance.
"I accuse Ennahda," the MP's sister Chhiba Brahmi told AFP at the family home in Sidi Bouzid. "It was them who killed him," she said, although she offered no evidence.
"Our family had the feeling that Mohamed would suffer the same fate as Chokri Belaid," whose family also blamed Ennahda, she added.
Ennahda chief Rached Ghannouchi rejected the charge in a statement to AFP. Brahmi's killing was "a catastrophe for Tunisia", he said.
"Those behind this crime want to lead the country towards civil war and to disrupt the democratic transition."
EU foreign policy chief Catherine Ashton condemned the killing, adding her voice to calls by UN human rights chief Navi Pillay, Amnesty International and Human Rights Watch for an investigation into the killing.
The United States condemned the "cowardly" assassination.
Brahmi was elected MP in October 2011 for Sidi Bouzid, birthplace of the revolution earlier that year that toppled Ben Ali.
On July 7, he resigned as general secretary of the Popular Movement, which he founded, saying it had been infiltrated by Islamists.
Following his killing, angry demonstrators took to the streets of Tunis to denounce the ruling Islamists.
Mohamed Maaroufi, a member of a youth committee that organised the protest, told AFP that they would stay in the streets until Ennahda had been forced from government.
In Sidi Bouzid, crowds, referring to the Muslim Brotherhood, chanted "Down with the Brothers, down with the people's torturers!"
Thousands also protested in nearby Menzel Bouzaine, where Ennahda party offices were set ablaze.
Prime Minister Ali Larayedh, himself an Islamist, told reporters: "I condemn in the strongest terms this odious crime which targets the whole of Tunisia and its security."
He also called for calm.
"This drama must not be exploited to sow trouble," he said. "Only minutes after news of the murder was announced, calls were made inciting Tunisians to kill each other."
President Moncef Marzouki said the killing was aimed at derailing the Arab Spring, and called it a "second national catastrophe" after Belaid's murder.
