At least 12 others, including several of al-Hariri's bodyguards, died when his motorcade was blown up as it passed through an upmarket section of Beirut's seafront, four months after he resigned as prime minister.
Former economy minister Basil Fulaihan, also riding in the convoy, was critically wounded. At least 100 other people were hurt, officials said.
The explosion outside the St George Hotel gouged a deep crater in the road, ripped facades from luxury buildings, and set cars ablaze on streets strewn with rubble and broken glass.
Vehicles from al-Hariri's convoy were torn apart despite their armour plating. A senior security source said the cause was a car bomb.
"Everything around us collapsed," a Syrian building worker at the site said. "It was as if an earthquake hit the area."
A previously unknown Islamist group said in a video aired by Aljazeera television it had carried out an attack against al-Hariri because he supported the Saudi government.
The Lebanese opposition blamed
Syrian and Lebanese officials
It added it was "the beginning of many martyrdom operations against the infidels and apostates in the Levant". The claim could not be confirmed.
Lebanese security forces said on Monday they had stormed the Beirut home of a man they identified as a Palestinian who appeared in a video claiming responsibility for the killing of a former Lebanese prime minister.
A Lebanese security source said Ahmad Abul Adas was not in the house. He had earlier appeared in a video aired by Aljazeera claiming responsibility for the killing of al-Hariri.
Act of terrorism
Al-Hariri had remained politically influential since his resignation and recently joined opposition calls for Syrian troops to quit Lebanon in the run-up to a May general election.
"Syria regards this as an act of terrorism, a crime that seeks to destabilise [Lebanon]," Syrian Information Minister Mahdi Dakhl-Allah told Reuters by telephone.
"Everything around us collapsed. It was as if an earthquake hit the area"
Witness to the blast
Syrian President Bashar al-Asad called the blast a "horrendous criminal act" and told Lebanese President Emile Lahud no effort should be spared to find the killers.
The White House condemned the killing and said Lebanon should be able to pursue its future "free from violence and intimidation and free from Syrian occupation", but added it did not know who killed al-Hariri and was not accusing Syria.
The White House added that it would consult with UN Security Council members about taking punitive measures against those responsible for the killing, and to push for an end to the Syrian presence in Lebanon.
Lebanon's anti-Syrian opposition accused the Syrian and Lebanese governments of responsibility for the assassination of al-Hariri and called for a three-day strike to protest his death.
At least 12 others were killed in
the blast on Beirut's seafront
"We hold the Lebanese government and the Syrian government, the power behind it, responsible for the crime," MP Basim Sabah said after an opposition meeting at Hariri's west Beirut family home.
Lebanese voices calling for Damascus to pull out its 14,000 troops have grown louder in recent months, backed by a UN Security Council resolution calling for their withdrawal.
Rescue workers clawed at piles of debris across the street from the hotel, which was closed for renovation. Witnesses said at least five people had been buried there by the explosion.
Scores of firefighters doused the burning vehicles and bloodied survivors were taken away by ambulance. Al-Hariri's body, with wounds and burns to the face, was taken to the American University Hospital where sympathisers gathered and wept.
Prime Minister Umar Karami, who visited the bomb scene surrounded by guards, was among many Lebanese politicians to condemn the attack. The Hizb Allah resistance group called it "a heinous crime" aimed at planting strife in the country.
"We hold the Lebanese and the Syrian government, the power behind it, responsible for the crime"
Lebanese MP Basim Sabah
Al-Hariri's funeral was planned for Wednesday, and the government called for three days of national mourning.
In October, a car bomb wounded opposition parliamentarian Marwan Hamada soon after he quit as economy minister in protest at the extension of pro-Syrian Lahud's term.
Beirut was often rocked by car bombs during the civil war, when fighting among religious and political factions all but tore Lebanon apart. But they have been rare since then.
Al-Hariri, 60, had held office for most of the past 12 years before quitting in October 2004 amid a bitter rift with Lahud.
The Sunni Muslim al-Hariri spent some 20 years in Saudi Arabia, where construction deals made him a fortune that Forbes estimated at $3.8 billion in 2003.
Businessmen praised him for rebuilding war-shattered Beirut, but hopes that an economic renaissance would flower with the Middle East peace process of the 1990s wilted with it instead.