Tunisia was hit by strikes Thursday as the murder of an outspoken opposition leader threatened to reignite violence despite the premier's attempts to defuse the crisis by pledging to form a new government.

Lawyers and judges across the country as well as teachers at Mandouba University, near Tunis, kicked off a two-day strike in response to the killing of Chokri Belaid outside his home on Wednesday, officials said.

The influential labour union, UGTT, meanwhile, was meeting to decide its course of action in the wake of the killing.

Police deployed in numbers in the capital's Habib Bourguiba Avenue, epicentre of the 2011 uprising that toppled ex-president Zine El Abidine Ben Ali, and where thousands had gathered Wednesday in scenes reminiscent of the revolution.

Security trucks, buses and vans were visible around the city centre and roads near the interior ministry were closed to traffic.

Shops reopened in the area, but many of them kept their shutters down to protect their windows.

While opposition parties and unions refrained from calling people onto the streets Thursday, spontaneous protests that erupted in a dozen towns and cities the previous day served as a reminder that social upheaval remains a real threat.

One policeman was killed after being hit by rocks in Tunis, while protesters torched and ransacked offices of the ruling Islamist Ennahda party in a number of towns as news spread of Belaid's assassination.

Ennahda has been squarely accused by Belaid's family of being behind the killing, which was carried out by a lone gunman -- charges it vigorously denies.

Prime Minister Hamadi Jebali, who hails from Ennahda, said in a televised address on Wednesday that he would form a new administration of non-political technocrats ahead of fresh elections.

"I have decided to form a government of competent nationals without political affiliation, which will have a mandate limited to managing the affairs of the country until elections are held in the shortest possible time," he said.

Jebali did not specify that he was dissolving the existing government, nor did he set a date for the reshuffle which must be confirmed by the national assembly.

Tunisia's ruling party has reportedly rejected moves to dismiss the government and form a cabinet of technocrats after the assassination of an opposition leader.

Ennahda's vice-president said Prime Minister Hamadi Jebali "did not ask the opinion of his party".

"We in Ennahda believe Tunisia needs a political government now," party Vice-President Abdelhamid Jelassi was quoted as saying.

"We will continue discussions with others parties about forming a coalition government," he added.

Tunisian media voiced fears that the murder of Belaid, a prestigious leftist opposition figure and outspoken critic of the ruling Islamists, could plunge the country into a new cycle of violence.

"The turn is extremely dangerous, as it is clear that nobody is safe now from death squads," said the daily Le Quotidien, which called on the government to "distance itself from the spectre of civil war."

La Presse said on Thursday that it was torn between "grief and hope".

"This is a welcome decision but a bit late," the paper said of Jebali's announcement, adding that "it took the death of a brave man" to push the government reshuffle that has been awaited for months.

Four opposition groups including Belaid's Popular Front bloc said they were pulling out of the National Constituent Assembly (NCA), which was elected in October 2011 but has failed to draft a new constitution.

The four groups blamed Interior Minister Ali Laraydeh from Ennahda for Belaid's murder and demanded his sacking "because he knew he was threatened and he did nothing," according to Nejib Chebbi, leader of one of the blocs.

"The government is no longer capable of managing the affairs of the country, just like the NCA. They must resign in the interests of the people, of Tunisia and of stability," Beji Caid Essebsi, a centre-right opposition leader and former premier told Shems FM radio station.

Belaid's family said his funeral will take place on Friday after the main weekly prayers.

The killing comes at a time of rising violence in Tunisia stoked by political and social discontent two years after the mass uprising that forced Ben Ali to flee and touched off the Arab Spring.

Militias close to Ennahda have, in particular, been accused of organising attacks on the secular opposition and trade unions, including beating to death a local opposition representative in a town in southern Tunisia in October.