BEIRUT (Reuters) - A huge car bomb Monday killed Lebanon's former Prime Minister Rafik al-Hariri, a billionaire who masterminded the country's reconstruction from its 1975-90 civil war.

At least 12 others, including several of Hariri's bodyguards, died when his motorcade was blown up as it passed through an exclusive section of Beirut's seafront, four months after he resigned as prime minister.

Former Economy Minister Basil Fuleihan, 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 Hariri's convoy were torn apart despite their armor 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 Al Jazeera television it had carried out a suicide attack against Hariri because he supported the Saudi government.

It added it was "the beginning of many martydom operations against the infidels and apostates in the Levant." The claim could not be confirmed.

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 destabilize (Lebanon)," Syrian Information Minister Mahdi Dakhl-Allah told Reuters by telephone.

Syrian President Bashar al-Assad called the blast a "horrendous criminal act" and told Lebanese President Emile Lahoud no effort should be spared to find the killers.

U.S. Condemnation

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 Hariri and was not accusing Syria.

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. Hariri's body, with wounds and burns to the face, was taken to the American University Hospital where sympathizers gathered and wept.

Prime Minister Omar Karami, who visited the bomb scene surrounded by guards, was among many Lebanese politicians to condemn the attack. The Shi'ite Hizbollah guerrilla group called it "a heinous crime" aimed at planting strife in the country.

Hariri's funeral was planned for Wednesday, and the government called for three days of national mourning.

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.

Neighboring Syria became ever more dominant during the conflict and took much of the credit for ending the war.

But Lebanese voices calling for Damascus to pull out its 14,000 troops have grown louder, backed by a U.N. Security Council resolution calling for their withdrawal.

In October, a car bomb wounded opposition parliamentarian Marwan Hamadeh, soon after he quit as economy minister in protest at the extension of pro-Syrian Lahoud's term.

Mohammad Jihad Ahmed Jibril, a Palestinian military leader, was killed by a bomb that ripped through his car in Beirut in May 2002. Earlier that year, a bomb killed Elie Hobeika, a key figure in a massacre of Palestinian refugees in 1982.

Political Rift

Hariri, 60, had held office for most of the past 12 years before quitting in October 2004 amid a bitter rift with Lahoud.

The Sunni Muslim 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.

"This is the work of an intelligence service, not a small group," said Rime Allaf, Middle East analyst at London's Royal Institute of International Affairs.