A leading opposition figure and critic of Tunisia's ruling Islamists, Mohamed Brahmi, was shot dead outside his home near the capital on Thursday in the second such assassination this year.
The murder by unknown gunmen sparked angry street protests in central Tunis and Brahmi's birthplace of Sidi Bouzid where he served as MP, AFP correspondents said.
"Mohamed Brahmi, general coordinator of the Popular Movement and member of the National Constituent Assembly, was shot dead outside his home in Ariana," 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 leftist movement, said in a tearful account aired on Tunisian radio.
The interior ministry, cited by TAP, said that Brahmi, a 58-year-old MP and vocal critic of Tunisia's ruling Islamists, was assassinated as he left home.
Watanya said Brahmi was struck by a hail of 11 bullets fired from point-blank range.
The February 6 assassination of Chokri Belaid, another opposition figure, in front of his home sparked a political crisis in Tunisia and charges of government involvement.
Condemning the latest killing, lawyer Mabrouk Korchid told AFP that Brahmi was "assassinated in cold blood on the day that Tunisia is marking" the 56th anniversary of the republic's declaration.
Brahmi, a man with a bushy moustache and weather-beaten complexion, was elected MP for Sidi Bouzid, birthplace of the 2011 revolution which toppled president Zine El Abidine Ben Ali.
On July 7, he resigned from his post as general secretary of the Popular Movement, which he founded, protesting that it had been infiltrated by Islamists.
"Tunisia is free, Brotherhood out!" angry demonstrators shouted in Tunis on Thursday, referring to the Muslim Brotherhood-linked Ennahda movement which heads the country's ruling coalition.
Ennahda chief Rached "Ghannouchi murderer!" they also chanted.
"This is a plot against the country, and the government must be held responsible for its lack of vigilance," said protester Fethi Mouelhi.
In Sidi Bouzid, crowds chanted "Down with the Brothers, down with the people's torturers!"
Thousands also took to the streets of nearby Menzel Bouzaine, where Ennahda party offices were set ablaze.
French President Francois Hollande, whose country was the pre-independence colonial power in Tunisia, strongly condemned Brahmi's killing and called on the country to unite behind its post-revolutionary democratic transition.
"The president condemns in the strongest terms the assassination this morning in Tunis of lawmaker Mohamed Brahmi," Hollande said in a statement.
He urged "all political and social forces in Tunisia to demonstrate more than ever the spirit of responsibility needed to preserve national unity and to guarantee the continuation of the democratic transition".
France's leader called for "light to be shed as quickly as possible" on both the murder of Brahmi and that of Belaid.
On Wednesday, a minister and senior adviser to the prime minister said six people believed to have orchestrated Belaid's killing have been identified.
Noureddin B'Hiri said after a cabinet meeting the details would be revealed "soon" by Interior Minister Lotfi Ben Jeddou, without saying when.