"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.
The February 6 assassination of Chokri Belaid, another opposition figure, in front of his home sparked a political crisis in Tunisia.
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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.