Al-Hariri's motorcade was blown up on Monday as it passed along a section of the city's waterfront Corniche.
A previously unknown Islamist group later claimed responsibility for the killing of al-Hariri on Monday, in a video tape aired on Aljazeera.
"For the sake of our mujahidin brothers in Saudi Arabia ... we decided to implement the just execution of those who support this regime," a bearded man said on the tape. He sat in front of a black flag carrying the name Group for Victory and Holy War in the Levant.
Al-Hariri, who resigned from government last October, had recently joined calls by the opposition for Syria to quit Lebanon in the run-up to general elections in May.
Lebanon is due to hold elections in the spring, and the explosion took place as the country and neighbour Syria are under intense international pressure, particularly from the United States, over the dominant role of Damascus in the affairs of its smaller neighbour.
Al-Asad described the Lebanese
leader's killing as a 'criminal act'
Syrian President Bashar al-Assad called the blast a "horrendous criminal act". Lebanese President Emile Lahud called an emergency cabinet meeting.
Syrian officials condemned the killing, describing it as an act of terrorism.
Speaking to Aljazeera, Mahdi Dakhl Allah, Syrian information minister, said, "The assassination of al-Hariri is painful news. It is a black day in Syria, Lebanon and the entire Arab world."
He accused "enemies of Lebanon" of being behind the attack. "[The perpetrators] are those who do not want stability, national unity and power in Lebanon, particularly because this action comes amid international pressure on Lebanon and Syria."
Responding to suggestions of a Syrian hand in the assassination, Dakhl Allah said such accusations were "ridiculous" and "aimed at misleading the world through the media".
Women mourn in Lebanon after
hearing news of al-Hariri's death
"Syria has good relations with all Lebanese people, including the opposition. Therefore, these accusations are incorrect and aim at distracting the world away from the real perpetrators of this crime."
In Washington, the White House condemned the killing and said Lebanon should be able to pursue its political future "free from violence and intimidation and free from Syrian occupation".
But spokesman Scott McClellan said the US did not know who was behind the bombing and he was not accusing Syria.
For his part, French President Jacques Chirac, a close friend of al-Hariri, called for an international inquiry into attack that killed him.
The European Union urged Lebanon to go ahead with the election in May, despite al-Hariri's assassination.
Earlier, Aljazeera's office in Beirut received a phone call from a person claiming to be speaking on behalf of a group called al-Nasir and Jihad Group in al-Sham countries [Syria, Lebanon, Jordan and Palestine].
Al-Hariri recently joined calls by
the opposition for Syrian pullout
The caller said the group has "announced carrying out the fair penalty against the infidel agent Rafiq al-Hariri", adding that it was a "martyrdom operation" whose details would be announced later.
"We have never heard about this group before," Ghassan bin Jiddo, director of Aljazeera's office in Lebanon, said. "The person is not a native-Arabic speaker. He was speaking Arabic with a foreign accent."
The situation is very tense in Beirut, Jiddo said.
"Demonstrations have been staged in Beirut streets, particularly al-Hamra Street," he said. "Slogans have been chanted in support of al-Hariri," he added.
Two former ministers, Samir al-Jisr and Basil Flaihan, who had been reported dead in the explosion, are both still alive, al-Jisr and aides of Flaihan said.
Al-Jisr, 61, a former education minister in the governments of al-Hariri, rang local media to say he was alive and well after the blast which killed his longtime boss.
"These accusations [against Syria] are incorrect and aim at distracting the world away from the real perpetrators of this crime."
Mahdi Dakh Allah,
Syrian Information Minister
Flaihan, a former economy minister in his 40s, was wounded but not killed, aides said.
Flaihan's wife, who was in Geneva, said she had received a phone call saying her husband had been taken to hospital but had no immediate word on his condition.