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 secular opponent of the moderate Islamist-led 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.
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 the Arab Spring uprising of January 2011, in a country where political assassinations are rare.
Speaking in front of the European Parliament on his visit to Strasbourg, President Marzouki said the murder of Mr Belaid should not affect Tunisia's revolution.
"There are many enemies of our peaceful revolution. And they're determined to ensure it fails," he said.
Referring to Mr Belaid as a "long-standing friend", he said his "hateful assassination" was a threat.
"This is a letter being sent to us that we will refuse to open. We reject that message and we will continue to unmask the enemies of the revolution," said the president, who was to participate in the summit of the Organisation of Islamic Co-operation in Cairo on Thursday and is instead returning home directly from Strasbourg.
According to AFP news agency, people torched the premises of the Ennahda party in the central town of Mezzouna, and ransacked the party's offices in the mining town of Gafsa in protest at Mr Belaid's death.
In Tunis, police fired tear gas to disperse protesters who had gathered outside the interior ministry, it reports.
Crowds had been chanting they want a "second revolution", the BBC's Sihem Hassaini in the city says.
Police also fired tear gas at demonstrators in Sidi Bouzid, the town where the revolution that toppled Zine el-Abidine Ben Ali from the presidency began a little more than two years ago, AFP reports.
A resident, Mehdi Horchani, told Reuters news agency that more than 4,000 people protested in the city, burning tyres and throwing stones at police.
"There is great anger," he is quoted as saying.
It is not known who is responsible for the attack on the politician.
Prime Minister Hamadi Jebali said his murder was an "act of terrorism" and promised to pursue all efforts to "immediately" arrest the murderer.
Mr Belaid was the co-ordinator of the left-leaning Democratic Patriots party, part of a group of opposition parties which has been challenging the government since it came to power following the country's first post-Arab Spring election in October 2011.
"This murder robs Tunisia of one of its most courageous and free voices," French President Francois Hollande said in a statement.
On Saturday, Mr Belaid accused "mercenaries" hired by the Ennahda party of carrying out an attack on a Democratic Patriots meeting.
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.