Former Lebanese prime minister Rafik al-Hariri and nine others were killed in a bomb attack on his motorcade in Beirut on Monday.
Al-Hariri, a billionaire businessman who resigned from the government last October, recently joined calls by the opposition for Syria to quit Lebanon in the run-up to the general election in May.
The powerful blast, which left 100 others wounded, occurred at 1000 GMT (1530 IST) in front of the five-star St George Hotel in the Lebanese capital's famed Cornische overlooking the Mediterranean Sea.
A plume of black smoke rose over the downtown. The blast shattered windows in the city's financial centre about a mile away.
The blast blew off the facade of at least one large building, covering cars, sidewalks and the street with rubble.
Heavily armed security forces cordoned off the area as rescue workers and investigators combed the scene looking for casualties or clues to what caused the explosion.
Monday's explosion occurred amidst increasing political tension ahead of the general election.
Lebanon's pro-Syrian government, which counts on its neighbour for security, has come under fire from the anti-Syrian opposition.
Syria maintains about 16,000 troops in Lebanon, based mainly east of Beirut and in the Bekaa Valley.
After Israel withdrew from southern Lebanon in 2000, the opposition demanded Syria to withdraw. The UN Security Council adopted a resolution last October calling on Syria to withdraw.
The 15-year civil war mostly pitted Lebanon's ruling conservative Christians against leftist Muslims, with Syria, Israel and Western international forces -- including US Marines -- occasionally taking part.
In October, a car bomb seriously injured an opposition politician and killed his driver in Beirut amid rising tension between the opposition and the government.