Zoubeir Souissi / Reuters
A forensic inspector looks at the car of opposition politician Chokri Belaid, who was shot dead outside his home in Tunis on Wednesday.
Updated at 12:08 p.m. ET: Tens of thousands of Tunisians took to the streets Wednesday to protest the apparent assassination of secular opposition leader Chokri Belaid, witnesses and local media reported.
Belaid, a staunch opponent of the moderate Islamist-led government, died after he was shot in the head and chest outside his home in Tunis.
The Interior Ministry said a man fired at Belaid then jumped onto a waiting motorcycle, which sped away.
Unrest built throughout the day, with the secular opposition Popular Front and it allied opposition members eventually saying they would pull out of the assembly that is acting as Tunisia's parliament and is charged with writing a constitution. The Popular Front also called for a general strike.
Fethi Belaid / AFP - Getty Images, file
Tunisian politician Chokri Belaid, seen in this file image, was assassinated early Wednesday.
Prime Minister Hamadi Jebali described the killing as a “political assassination” and a blow to the country’s 2011 Arab Spring revolution, Reuters reported. “By killing him they wanted to silence his voice," Jebali said.
The victim’s brother blamed the killing on the ruling party, Ennahda, of which Jebali is a member. The party's headquarters was later set on fire.
"I accuse [Ennahda leader] Rached Ghannouchi of assassinating my brother," Abdelmajid Belaid told AFP. "My brother was assassinated. I am desperate and depressed."
France24 reporter David Thomson posted a picture on Twitter of Belaid's angry wife clutching bloodstained clothing.
The ruling party, however, vehemently denied involvement.
"Ennahda is completely innocent of the assassination of Belaid. … Is it possible that the ruling party could carry out this assassination when it would (only) disrupt investment and tourism?" party President Rached Gannouchi told Reuters in an interview.
"Tunisia today is in the biggest political stalemate since the revolution. We should be quiet and not fall into a spiral of violence. We need unity more than ever," he added.
Images posted on Twitter by English-language news source Tunisia Live showed angry crowds facing riot police on the streets of Tunis on Wednesday.
Witnesses told Reuters that thousands of protesters had gathered there and in Sidi Bouzid, cradle of the Arab Spring uprisings.
Protesters were "burning tires and throwing stones at the police," said Mehdi Horchani, a Sidi Bouzid resident. "There is great anger."
Police responded by firing shots in the air and using teargas, both in Sidi Bouzid and in Tunis, where an estimated 20,000 protesters had massed outside the Interior Ministry.
Authorities scattered the Tunis protesters as an ambulance carrying Belaid's body approached, Reuters reported.
Tunisians rose up against long-time leader Zine al-Abidine Ben Ali after vegetable seller Mohamed Bouazizi set fire to himself in Sidi Bouzid in late 2010.
President Moncef Marzouki cut short a visit to France to return to Tunisia following Wednesday’s killing, according Tim Marshall, diplomatic editor of U.K. news channel Sky News.
Tunisia -- the first Arab country to oust its leader and hold free elections -- had made a relatively smooth transition to democracy.
However, it has recently been plagued by economic hardship and the threat from al-Qaida-linked militants.
Reuters contributed to this report.