Former Lebanese Prime Minister Rafik Hariri was "almost certainly" killed by a suicide bomb, initial findings show.
The killing has caused shock and fear across Lebanon
The US has recalled its ambassador to Syria for consultations over the death, which raised fears of a return to the violence of Lebanon's civil war years.
The United Nations Security Council called for the perpetrators to be brought to justice.
And UN Secretary General Kofi Annan has urged more progress on the withdrawal of Syrian forces from Lebanon.
Before leaving Damascus, US Ambassador Margaret Scobey delivered a note to the Syrian government expressing US outrage over the killing.
State department spokesman Richard Boucher said the US was not blaming Syria directly for Mr Hariri's death.
But he said the incident underlined what he called the distortions caused by the presence of 14,000 Syrian troops in Lebanon.
"Yesterday's bombing calls into question the stated reason behind this presence of Syrian security forces: Lebanon's internal security," Mr Boucher said.
Mr Annan said he had recently written to Syrian President Bashar al-Assad, to urge him to comply with a UN resolution calling for the withdrawal of foreign troops from Lebanon.
Mr Annan was speaking before a meeting of the Security Council, which unanimously condemned the bombing.
In a statement, the council said it was concerned about the impact it could have on Lebanese elections due in May.
A bomb blew up close to Hariri's motorcade in Beirut on Monday, killing at least 15 people and injuring about 120 others.
The Lebanese army was put on high alert after the blast.
Shops, schools and public institutions across the country are closed for three days of official mourning.
"The security services are almost certain that it was a suicide car bomb," said Interior Minister Suleiman Franjieh.
Prime minister 1992-98 and 2000-04
Trained as a teacher, but founded successful construction firm in Saudi Arabia
Born in 1944 to a poor Sunni Muslim family in the southern Lebanese port of Sidon
Lebanese opposition figures have blamed Hariri's death on the government and Syria, and have renewed calls for Syrian troops to withdraw.
Hariri had called on Syria to end its involvement in Lebanese affairs.
The Syrian government has denied involvement.
Hours after the attack, a little-known group calling itself Victory and Jihad in Greater Syria claimed responsibility.
However, correspondents say the statement could have been a ploy by those responsible to draw suspicion away from themselves, or by fringe elements trying to capitalise on the attack.
Many Beirut residents stayed at home on Tuesday.
Traffic was almost non-existent and the only sound was that coming from a mosque where prayers were already being said for Mr Hariri.
Grief turned to anger among Mr Hariri's supporters, and mourners in the capital tried to torch the local offices of the Syrian Baath Party.
In Mr Hariri's hometown of Sidon, protesters burned tyres and blocked roads.
His funeral will be held on Wednesday at a mosque in central Beirut.
His family has called for a large popular turnout, but it has also said it does not want members of the government to attend.