Tunisian opposition politician Chokri Belaid has been shot dead outside his home in the capital, Tunis.
Relatives say Mr Belaid was shot in the neck and head on his way to work.
He was a prominent opponent of the government and his murder has sparked protests around the country, with police firing tear gas to disperse angry crowds.
President Moncef Marzouki has condemned the killing and is cutting short a visit to France to return home.
This murder robs Tunisia of one of its most courageous and free voices”
He has also cancelled a scheduled appearance at a summit in Egypt to return home.
Tunisia is currently gripped by 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.
BBC Arabic’s Wafa Zaiane says this is the first time a political leader has been assassinated since 2011, in a country where political assassinations are rare.
Born in the Jabal al-Joloud area of Tunis in November 1964
A lawyer and human rights activist, he was also the co-ordinator of the left-leaning Democratic Patriots party
One of Tunisia’s most prominent political figures and an outspoken critic of the government
Even though his party did not win many seats in the 2011 elections, he remained an important opposition figure who warned against the escalating violence
The Paris-based France 24 TV station has reported that Mr Belaid reportedly received recent death threats.
It said that he died in hospital after being shot by “three men in a black vehicle”.
“My brother was assassinated. I am desperate and depressed,” Mr Belaid’s brother Abdelmajid Belaid told AFP.
Correspondents say that although Mr Belaid’s party did not have a large share of the election vote, it spearheaded popular concern over the rising level of political violence in Tunisia.
M.D