A massive bomb has torn through former Prime Minister Rafik Hariri's motorcade, assassinating the billionaire businessman who helped rebuild his country after decades of war.
At least nine other people were killed and 100 wounded, including a former economy minister.
The blast raised immediate fears that Lebanon - largely peaceful since the 1990 end of its civil war - was headed toward a new and bloody twist in its divisive dispute over the role of Syria, which maintains about 15,000 troops in the country.
The Lebanese National News Agency, quoting a statement from American University Hospital, said Hariri was pronounced dead on arrival, his body mutilated.
Some 300 kilograms of TNT explosives were used in the bombing, security officials said on condition of anonymity.
They did not say whether the explosives were placed in a vehicle or on the side of the street.
Following an emergency Cabinet meeting, Information Minister Elie Ferzli told reporters that government will refer the case to the Higher Judiciary Council that looks into political and state security crimes.
Lebanon's Supreme Council for Defence instructed the army and internal security forces "to take all necessary measures to control the security situation."
The council, which groups the president, Cabinet ministers and military officials, declared three days of national mourning.
President Emile Lahoud, a long time rival of Hariri, issued a statement saying the killing was "a dark point in our national history."
He promised the perpetrators would be brought to justice.
There were no credible claims of responsibility.
However, a previously unknown group, calling itself Support and Jihad in Syria and Lebanon, said it had carried out the bombing.
It said the attack was a suicide operation and would be followed by more attacks "against infidels, renegades and tyrants."
The claim, which could not be authenticated, appeared in a video aired on al-Jazeera satellite television station.
US President George W Bush was "shocked and angered" by the assassination, said White House press secretary Scott McClellan.
McClellan said the White House did not know who was responsible for the attack, but whoever is behind the bombing should be punished.
He said the assassination was "a terrible reminder that the Lebanese people must be able to pursue their aspirations and determine their own political future, free from violence, and intimidation and free from Syrian occupation."
In Paris, Lebanon's most prominent exile, former army commander General Michel Aoun, was more direct, blaming Syria and what he called the "feeble regime imposed by Syria" for the assassination.
"I think all these together are behind this crime," Aoun told al-Arabiya satellite television, which is based in Dubai.
Some Lebanese regularly accuse Syria of being behind violent attacks in the country, dating back to civil war days.
Syria's allies, in turn, tend to blame Israel for all violence.
On Sunday, one day before the assassination, the pan-Arab newspaper al-Hayat quoted an unidentified source as saying UN envoy Terje Roed-Larsen had warned Syrian President Bashar Assad during a Thursday meeting in Damascus against harming any Lebanese opponents of Syrian involvement in the country - mentioning Hariri and Druse leader Walid Jumblatt by name.
Al-Hayat quoted its source as saying Roed-Larsen had warned any such bloodletting would "constitute a point of no return in the relations between Syria and the international community." Roed-Larsen could not immediately be reached for comment.
Foremost among the wounded was former Economy Minister Bassel Fleihan, a member of parliament in Hariri's bloc, who was admitted to the intensive care unit of the American University Hospital, said another pro-Hariri legislator, Atef Majdalani.
Hariri's own Future TV reported that Fleihan was in critical condition and the hospital was preparing to transfer him abroad.
Hariri's assassination removes a main political buffer in a country divided among an opposition strongly opposed to Syria's role, and the pro-Syrian government camp. He was killed after attending a parliamentary debate where the two camps are divided over a new election law for polls due in April and May.
Hariri's supporters quickly took to the streets, chanting his praises outside the American University Hospital where he was declared dead. In his hometown of Sidon, supporters blocked roads and burned tires.
Hariri is a self-made billionaire who led Lebanon for 10 of the years since the end of the 1975-1990 civil war. He was elected prime minister in 1992 and served until 1998, forming at least three Cabinets. He was elected again in 2000 and served until he resigned in October.
Hariri moved toward the opposition camp after leaving office - in large part because of a dispute concerning Syria's controversial role in Lebanon. Hariri had rejected a position supported by Syria that his old rival, President Lahoud, remain in office as president for three more years.
Pro-Syrian allies of Lahoud accused Hariri of being behind the UN Security Council resolution in September that demanded Syria withdraw its army from Lebanon and stop interfering in the country. The resolution was sponsored by the United States and France.
Hariri was credited with rebuilding Lebanon from the destruction of the civil war, but he was faulted for shackling Lebanon with a debt of more than US$35 billion.
His wide international business and political connections helped earn Lebanon wide recognition and attracted badly needed foreign investment.
A large blast on the seafront of west Beirut, Lebanon has killed several people, including former Prime Minister Rafik Hariri.

Current reports conflict on the total number of casualties, suggesting nine or ten people killed and around 100 injured. Hariri was travelling in a car in a motorcade when the explosion occurred around 1pm local time. His bullet-proof car was totally destroyed. The attack took place near the luxurious Saint George hotel, in a residential neighborhood.

The cause of the blast is currently unknown for certain but it is believed to have been a car bomb. Anonymous sources have said car containing 300 kg of TNT was used; however other reports have suggested the explosives were placed in a building. The explosion created a crater at least 10 m across and devastated the fronts of nearby shops and hotels, destroying several buildings entirely. Apartment windows within a 1km radius were blown out.

Only one group has claimed responsibility, a little-known organisation from Syria called Victory and Jihad in Greater Syria. There has been no confirmation that this group actually carried out the attack. Hariri's supporters see Syria as being responsible for the attack. Lately, Hariri had become a chief opponent of the Syrian military presence (14,000 troops) in Lebanon.

The BBC quotes the group has having said "We hold the Lebanese authority and the Syrian authority, being the authority of tutelage in Lebanon, responsible for this crime and other similar crimes."

Rafik Bahaa Edine Hariri (November 1, 1944–February 14, 2005) was a Lebanese self-made billionaire businessman, and was twice Prime Minister of Lebanon, before his last resignation from office on October 20, 2004.