Tunisian opposition politician Mohamed Brahmi has been shot dead outside his home in Tunis, the capital, officials say.
Mr. Brahmi, 58, led the nationalist Movement of the People party.
It is the second time an opposition party leader has been killed this year.
In February, prominent secular politician Chokri Belaid was also shot outside his house in Tunis. His murder sparked protests and forced Prime Minister Hamadi Jebali to resign.
An uprising in Tunisia in late 2010 kick-started a series of revolutions that spread through the Middle East and became known as the Arab Spring.
Gunmen on a motorbike shot Brahmi in his car in front of his wife and daughter on Thursday morning, Movement of the People party officials said.
Local media said the assailants fired 11 bullets at the politician. It is not known yet who was behind the attack.
Brahmi's wife, Mbarka, said a "criminal gang has killed the free voice of Brahmi".
She and her daughter Belkis were joined by angry Tunisians outside the hospital in Tunis where the politician died.
Large crowds also gathered outside the Ministry of Interior in Tunis in protest at the killing.
There were also reports of demonstrators converging in the city of Sidi Bouzid, Brahmi's hometown and the birthplace of the Arab Spring.
"People have blocked roads and set tyres alight," a local resident told the Reuters news agency.
The governing Islamist Ennahda party expressed "sadness and shock" at the assassination, which it described as a "cowardly and despicable crime".
The killing came as Tunisia celebrated the 56th anniversary of becoming a republic after gaining independence from France.
French President Francois Hollande condemned Brahmi's murder and called on Tunisians to unite behind the democratic transition.
Brahmi founded the Movement of the People party after the 2011 revolution.
He was also a member of the National Constituent Assembly, which is drafting a new constitution.
The assembly announced Friday would be a day of mourning.
Brahmi was not as big a political figure as Mr Belaid, but he too was a leftist critical of Ennahda.
Ennahda came to power following the overthrow of long-term ruler Zine al-Abidine Ben Ali in January 2011.
The party has faced growing popular unrest over a faltering economy and a rising extremist Islamist movement.
After Mr Belaid's assassination in February, many Tunisians accused Ennahda of not doing enough to stamp out a rise in Islamist violence, with some critics saying the party was actively fomenting it, correspondents say.
The party denied the accusations.
The Punch
There is so much assasination in politics nowadays all over Africa and it does not make any sense. That is not how things should be.