Tunisian opposition politician Chokri Belaid has been shot dead outside his home in the capital, Tunis, his brother and officials say.
They say he was shot in the neck and head on his way to work, reports the BBC.
Mr Belaid was a prominent secular opponent of Tunisia's moderate Islamist-led government.
About 1000 people gathered outside the interior ministry in Tunis to protest against the assassination.
Prime Minister Hamadi Jebali said his murder was an "act of terrorism".
The BBC says Tunisia is gripped by a political crisis as talks on a long-awaited cabinet reshuffle to include a wider range of parties in a coalition led by the Ennahda party have broken down.