TUNIS, Tunisia — Thousands of Tunisians mourned slain opposition leader Chukri Beleid Friday as police fired tear gas in clashes with groups of young men during his funeral, heightening tensions as the country faces its most serious crisis since the 2011 uprising that began the "Arab Spring."

At least four cars were torched in the unrest outside the Jalaz cemetery in the capital, Tunis, sending plumes of black smoke pillowing above the skyline. Clouds of tear gas forced mourners to cover their faces as they waited for the cortege.

The youths had been trying to steal cars and threw stones after being confronted by police. Two military helicopters hovered as mourners waved Tunisian flags and chanted “down with the regime.”

They carried photos of Beleid, a 48-year-old human-rights lawyer who had criticized the ruling moderate Islamist Ennahda party and radical religious groups.

Police also fired tear gas at looters on the capital’s main Avenue Habib Bourguiba. The General Union of Tunisian Workers called on its members to strike to mark the funeral. Protests by lawyers, judges and some teachers began yesterday.

All flights to and from Tunisia were canceled for the day, Al-Arabiya television reported, citing airport authorities.

Accusations by Beleid’s widow and other opposition figures that the ruling Islamist Ennahda colluded with extremists in the assassination, and violent anti-government protests that followed, sparked the most unrest since the ouster of President Zine El Abidine Ben Ali.

Government’s Demise?

Four secular opposition parties withdrew from the government after the Feb. 6 murder. Prime Minister Hamadi Jbeli of Ennahda vowed to form a cabinet of technocrats, a plan rejected by his own party, which said he was acting unilaterally.

The proposal, in exposing divisions within Ennahda, will probably “accelerate the current crisis and eventually lead to the fall of the government,” New York-based Eurasia Group, a political risk-analysis group, said late yesterday in an emailed note.

The political uncertainty and instability may last for weeks before the parties within the majority coalition work out an acceptable compromise, it said.

Two days of clashes between security forces and demonstrators that followed the killing left one policeman dead.

Police closed down central Tunis yesterday and fired tear gas at protesters. Shops were shut on Avenue Habib Bourguiba, where crowds outside the Interior Ministry shouted “down with the Brotherhood Party, executioner of the people” and “martyr rest in peace, we will continue to struggle.”

IMF Talks

The assassination sent Tunisia’s benchmark stock index tumbling 3.7 percent on Feb. 6, the biggest drop since Jan. 13, 2011, the eve of Ben Ali’s departure.

The killing also reverberated in Egypt, where secularists have expressed alarm over the power of Islamists under President Mohammed Morsi. The Tunis index rose 0.7 percent as of 2:30 p.m. in Tunis.

Tunisia in talks for a $1.78 billion International Monetary Fund stand-by loan to help buttress an economy that grew 2.7 percent in 2012 and may expand 3.3 percent this year, according to IMF estimates.

Beleid’s widow, Besma Khalfaoui, said yesterday that she held Ennahda, “with its fascist and democratic branches,” responsible for his death, a charge the party denied. She said her husband had asked police for protection.

Jbeli and the secular president, Moncef Marzouki, condemned the assassination, urging Tunisians not to be dragged into violence.

Salafi Cleric

Tunisia’s road to democracy has been paved with rifts and accusations that the influence of a small yet vocal and at times violent movement of ultraconservative Muslims known as Salafis may undercut a secular tradition.

Beleid’s murder came after a Salafi cleric said in a video posted on the Internet that his head, and that of Ahmed Najib Chebbi, another top opposition party figure, were “wanted.”

His death followed a Feb. 3 arson attack on the secular Nidaa Tounes opposition party headquarters in Kebili province, and clashes between opposition groups and government supporters in the cities of El Kef and Kairouan.

Hussein Abbasi, secretary-general of the Tunisian labor union organizing today’s strike, received a death threat yesterday on his mobile phone, union spokesman Sami Tahiri said.

In Washington, State Department spokeswoman Victoria Nuland said demonstrators and the Tunisian government should exercise restraint and called for a full investigation of Beleid’s death.

The assassination drew quick rebuke from secularists in Egypt, where Hosni Mubarak’s 2011 ouster unleashed unrest and sent the economy into a tailspin. They say Morsi has put the interests of the Muslim Brotherhood organization that fielded him for office ahead of the nation’s.

Egypt’s Cabinet said yesterday it may take legal steps against anyone who issues fatwas inciting violence. The statement came a day after Nobel Laureate Mohamed ElBaradei, an opposition leader, criticized the government for its silence.