Pierre Gemayel, a leading anti-Syrian Lebanese minister and Maronite Christian leader, has been killed in the capital, Beirut.
Gemayel's death is the latest in a series of assassinations in Lebanon
Mr Gemayel, 34, was shot in his car in a Christian suburb and rushed to hospital, where he died.
His death comes amid a political crisis in Lebanon, following the resignation of six pro-Syrian cabinet members.
Lebanese Prime Minister Fouad Siniora said such killings would not intimidate the country.
"Assassinations will not terrorise us," he told a press conference. "We will not let the criminal killers control our fate."
Three days of nationwide mourning for Pierre Gemayel will be observed in Lebanon beginning on Wednesday, with his funeral due to be held on Thursday.
Mr Gemayel, minister for industry, was a member of the Phalange Party and the son of former President Amin Gemayel.
At least three gunmen rammed Mr Gemayel's car with their vehicle before spraying it with gunfire and shooting Mr Gemayel at point blank range, Reuters news agency quoted witnesses as saying.
Television pictures from the scene of the killing in the Jdaideh neighbourhood showed Mr Gemayel's car peppered with bullet holes and with its bonnet crumpled.
Throngs of angry supporters of Mr Gemayel gathered at the hospital where he was taken, some crying and screaming.
Speaking to the crowds there, Amin Gemayel appealed for calm.
"I call on all those who appreciate Pierre's martyrdom to preserve his cause and for all of us to remain at the service of Lebanon. We don't want reactions and revenge," he said.
As news of the killing spread, streets emptied and the Lebanese army deployed at key areas across and just outside the capital, the BBC's Beirut correspondent Kim Ghattas reported.
Protests were reported in Christian areas of Beirut, as well as in the Christian town of Zahle in eastern Lebanon and in Mr Gemayel's hometown of Bikfaya, east of the capital.
BBC Middle East correspondent Jeremy Bowen says that this assassination will re-awaken Lebanon's worst fear, the fear of a return to civil war, because it seems to have been designed to inflame sectarian tensions at a time when Lebanon was already going through a profound political crisis.
The news of the killing was confirmed by Saad Hariri, the son of the country's former Prime Minister Rafik Hariri, who was himself killed last year in a huge blast.
On Tuesday, the United Nations Security Council approved plans for a special international tribunal to try those accused of killing the former prime minister in 2005.
A recent UN report implicated Syria in Rafik Hariri's death, although Syria has denied involvement.
The car in which Gemayel was travelling was sprayed with gunfire
Saad Hariri, who is also the leader of the anti-Syrian coalition which has a majority in parliament, accused Damascus of being behind the killing of Mr Gemayel as well.
"The Cedar Revolution is under attack...Today one of our main believers in a free democratic Lebanon has been killed. We believe the hand of Syria is all over [this]," he told CNN.
Syria denounced Mr Gemayel's assassination, calling it a "despicable crime".
"Syria strongly condemns the killing," the official Syrian news agency, Sana, said.
Western condemnation of Mr Gemayel's murder was swift.
US President George W Bush called for a full investigation to identify "those people and those forces" behind the killing.
Mr Bush reiterated Washington's support for Mr Siniora's government, saying he supported the Lebanese people's "efforts to defend their democracy against attempts by Syria, Iran and allies to foment instability and violence in that important country".
UK Prime Minister Tony Blair said Tuesday's killing was "completely without any justification at all.
"We condemn this murder utterly."
Six pro-Syrian ministers quit their jobs last week in an attempt to bring down the government after their calls for a greater role were rejected.
Despite the resignations, the depleted cabinet approved UN plans for setting up an international tribunal to try suspects in the Hariri killing.