Mohamed Brahmi, a prominent Tunisian opposition figure, has been shot dead in the capital Tunis.
The leader of the secular nationalist Popular Front party, he was a forceful critic of the Islamist-led government.
No-one has claimed responsibility for the killing, which has already drawn protests.
The 58-year-old was gunned down by two men on a motorbike outside his house. The killers then escaped.
It follows the murder in February of Chokri Belaid from the same party.
Some protesters are blaming the ruling Ennahda party and its leader for Brahmi’s death.
“Ghannouchi you’re a murderer. Oh Chokri Belaid, Today you host a new martyr,” cried lawyer and human rights activist Leila Ben Debba.
Mohamed Brahmi’s son said his father had died for the sake of his country.
“You, Tunisians, you must die for the thing he died for, die for the thing he died for. That’s it I cannot add any more,” he said from outside the room where the body of his dead father lay.
“On the day everybody agreed on the principle of a new Republic, a criminal sneaks to his house with militias and kills our brother Mohamed Brahmi. These militias killed him with bullets in the head and many others in the stomach,” said MP Noomen El Fehri.
The murdered politician was a member of the Constituent Assembly charged with drawing up a new Tunisian constitution.
Ennahda has condemned the killing. That has not deterred protesters in Tunis and Brahmi’s home town from calling on the government to resign.
The airline Tunisair said it was cancelling all flights on Friday after the country’s largest union called a general strike in protest at the assassination.