The murder of the outspoken government critic on Wednesday prompted President Moncef Marzouki to cut short a foreign tour, one of his advisers, Ghassen Dridi, told Agence France-Presse (AFP). The presidency urged "restraint and wisdom".
Prime Minister Hamadi Jebali denounced the murder as an "act of terrorism" while the presidency in a statement dubbed it an "odious" crime designed to "lead the Tunisian people to violence".
Protests erupted in several Tunisian towns and demonstrators torched one Ennahda party office and ransacked another, as Belaid's brother accused the Islamist Ennahda party of being behind the assassination.
At least 1 000 people protested outside the interior ministry in Tunis shouting abuse at Ennahda, AFP correspondents said.
Jebali, from the Ennahda party, said a lone gunman wearing the traditional hooded, long burnous robe shot Belaid with three bullets fired at close range as he left his Tunis home on Wednesday morning.
"This is a criminal act; an act of terrorism not only against Belaid but against the whole of Tunisia," Jebali told private radio station Mosaïque FM, while promising to do everything possible to swiftly arrest the murderer.
"The Tunisian people are not used to such things. This is a serious turn … our duty to all, as a government, as a people, is to be wise and not fall into the criminal trap which seeks to push the country into chaos."
'My brother was assassinated'
France condemned the murder, describing Belaid as a courageous fighter for human rights.
"This murder robs Tunisia of one of its most courageous and free voices," President Francois Hollande said.
The family of Belaid, who headed the opposition Democratic Patriots party and was a harsh critic of Tunisia's Islamist-led government, was in no doubt as to who was behind the murder.
"My brother was assassinated. I am desperate and depressed," said Abdelmajid Belaid.
"I accuse [Ennahda leader] Rached Ghannouchi of assassinating my brother," he said. Ghannouchi later emphatically denied involvement.
Belaid's wife told told Mosaïque FM her husband was hit by two bullets as he left home.
The murder of Belaid comes at a time when Tunisia is witnessing a rise in violence fed by political and social discontent two years after the mass uprising that toppled ex-dictator Zine El Abidine Ben Ali.
Several opposition parties and trade unions have accused pro-Islamist groups of orchestrating clashes or attacks against them.
The killing fuelled anger across Tunisia.
Around 1 000 protesters massed outside the interior ministry on Habib Bourguiba Avenue – epicentre of the 2011 uprising that ousted Ben Ali – shouting anti-Ennahda slogans and singing the national anthem, journalists reported.
Demonstrators torched Ennahda offices in Mezzouna near Sidi Bouzid and ransacked Gafsa in the centre of the country while protests rocked several cities, reporters and witnesses said.
Sidi Bouzid was where the desperate act of a jobless young man who set himself alight in late 2010 triggered the Tunisia uprising.
Marzouki scrapped foreign engagements to rush back home, his office said.
Marzouki was in Strasbourg on Wednesday morning, where he took part in a session of the European Parliament and met Hollande. He was due to fly later to Cairo to attend a summit of the Organisation of Islamic Co-operation.
Belaid's party belonged to the Popular Front coalition of leftist parties that has emerged in opposition to the Tunisia government. – AFP