While the government called for three days of mourning and a state funeral, Lebanese anti-Syrian opposition leaders demanded a three-day general strike, the resignation of the government and a Syrian troop withdrawal from Lebanon.
"We hold the Lebanese government and the Syrian government, the power behind it, responsible for the crime," MP Basim Sabah said on Monday after an opposition meeting at al-Hariri's Beirut family home.
"We demand the resignation of the government, which has lost all legitimacy, and the formation of a caretaker government," said Sabah, flanked by Druze leader Walid Jumblatt and other opposition figures.
He said Syria should pull out its 14,000 troops from Lebanon before parliamentary elections due in a few months' time.
Exiled Lebanese political leader and former prime minister Michel Aun bluntly blamed Syria.
Walid Jumblatt said Syria bore
responsibility for al-Hariri's death
"They are responsible. It's they who control the security and intelligence services" in Beirut, he said.
But Syrian President Bashar al-Asad condemned the assassination as a "terrible criminal act" and voiced solidarity with "brotherly Lebanon in this dangerous situation".
He urged the Lebanese people "to reinforce their national unity and reject all those who aim to cause trouble and sow division among the people", the official Syrian news agency Sana said in Damascus.
Lebanon's pro-Syrian President Emile Lahud said his political foe had died a "martyr for a united Lebanon", ordering three days of mourning and a state funeral for al-Hariri.
Syria's main regional ally Iran expressed concern about the fallout of what it condemned as a "terrorist act" - and cast suspicion on Israel.
"An organised terrorist structure such as the Zionist regime has the capacity for such an operation whose aim is to undermine the unity of Lebanon," foreign ministry spokesman Hamid Reza Asefi was quoted as saying by the official IRNA news agency.
"An organised terrorist structure such as the Zionist regime has the capacity for such an operation whose aim is to undermine the unity of Lebanon"
Hamid Reza Asefi, Iranian foreign ministry spokesman
The Lebanese army meanwhile announced a "general mobilisation to safeguard stability" in the country, recalled soldiers on leave and deployed troops in Beirut and other regions.
The Saudi cabinet sent "the kingdom's heartfelt condolences" and stressed its "total rejection of terrorist acts against innocents that seek to plant chaos and destruction".
Blaming Saudi Arabia
Prince Talal bin Abd al-Aziz, half-brother of King Fahd of Saudi Arabia, expressed doubt about the authenticity of an unknown group claiming responsibility for the massive bomb attack.
In a videotape broadcast by Aljazeera, a group calling itself al-Nusra wa al-Jihad fi Bilad al-Sham [Victory and Jihad in Palestine, Jordan, Lebanon and Syria] claimed responsibility for the attack.
The group said it carried out the attack on al-Hariri's convoy "because of his relations with the Saudi authorities" but provided no proof of its claim.
"I don't believe that what was transmitted on television about this group is true," Prince Talal told Aljazeera. "What does Saudi Arabia have to do with this affair?"
He said he did not believe what had been ascribed to the group, saying he thought it was a cover for another group.
"Who? ... God knows. But it's a cover," said the prince, who chairs the Gulf and Arab World Programme.
The blast killed at least 13 and
wounded more than 100
"I ask our friends in the Lebanese authorities not to reject this reasonable request," he said. "Our brothers in Lebanon must not throw accusations about. We must wait until the international community sets up an investigating committee to reveal the truth."
The US also condemned the killing of al-Hariri, vowing punitive action and calling for an end to Syria's military presence in Lebanon.
Shock and anger
US President George Bush "was shocked and angered" by the massive bombing that killed the former premier and at least nine other people, said White House spokesman Scott McClellan, who stopped short of blaming Damascus for the attack.
"This murder today is 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," he said.
At least 13 people were killed when an apparent car bomb went off as al-Hariri's motorcade passed through an upmarket section of Beirut's seafront. At least 100 others were wounded.
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.