The move thrust the country into its worst political crisis since 2008 and sparked fears of sectarian Shiite-Sunni unrest.

The powerful Hezbollah and its allies withdrew from the government formed in November 2009 as Hariri was in Washington holding talks with U.S. President Barack Obama on the crisis.

The 40-year-old Hariri, who now heads a caretaker government, made no comment after the announcement and headed to France where he was to meet on Thursday with President Nicolas Sarkozy.

According to the constitution, Lebanese President Michel Suleiman should start consultations with members of parliament to nominate a prime minister to form a new government.

But political sources said no steps will be taken before Hariri returns home from abroad and holds talks with Suleiman.

Previously, officials declined to say whether Hariri, whose coalition won a 2009 parliamentary election, will be asked to form a new government, or if someone else would be nominated.

Hezbollah, the only armed group not to disband after the 1975-90 civil war, is now the most powerful military force in Lebanon, stronger even than the army.

Hezbollah portrays itself as spearheading Islamic resistance to Israel, not as a sectarian group. That image would be badly damaged if it were proven to have had a role in the 2005 huge truck bomb that killed Hariri and 22 others.

While outright conflict may be unlikely, street protests, skirmishes or even a return to the bombings and political killings that followed the 2005 attack could not be ruled out, analysts said.

For months, Hezbollah, which is backed by Iran and Syria, has been pressing the Western-backed Hariri to disavow the special tribunal saying it is part of a U.S.-Israeli plot.

According to unconfirmed press reports, the tribunal is poised to indict senior Hezbollah members in connection with Rafiq Hariri's 2005 assassination, a move the militant party vehemently rejects.

The White House accused Hezbollah of toppling the government out of "fear" and reaffirmed its full support for Hariri and the tribunal.

France, Britain and U.N. Secretary General Ban Ki-moon also reiterated their unconditional support for the tribunal set up in 2007 following Hariri's murder and that of a number of anti-Syrian politicians.

Syria and Saudi Arabia have sought in recent months to mediate the crisis between Hariri and Hezbollah but diplomats said they would halt their efforts given the collapse of the government.

 Trying to bring the government down as a way to undermine the special tribunal is an abdication of responsibility, but it also will not work 

U.S. Secretary of State Hillary Clinton

Lebanese newspapers predicted a long drawn out crisis but there was no fear of an immediate breakout of violence.

"Hariri loses unity government... at Obama's doorstep," said the front-page headline of the Arabic-language daily As-Safir, close to Hezbollah.

"Lebanon entered a new phase yesterday, an open-ended one characterized by a profound and long-term political and government crisis," the paper said.

Hariri in recent days had discussed the crisis in New York with Saudi King Abdullah, U.S. Secretary of State Hillary Clinton, Sarkozy and Ban.

Clinton, who was in Qatar on Wednesday, said Hezbollah's attempt to undermine stability in Lebanon was bound to fail.

"We view what happened today as a transparent effort by those forces inside Lebanon, as well as interests outside Lebanon, to subvert justice and undermine Lebanon's stability and progress," Clinton said.

"Trying to bring the government down as a way to undermine the special tribunal is an abdication of responsibility, but it also will not work."

Egyptian Foreign Minister Ahmed Abul Gheit called on the parties to exercise "restraint" and to "act within the framework of (the country's) institutions to contain the crisis."

The standoff between Hariri's camp and Hezbollah had virtually paralyzed the government since its creation and prompted fears of sectarian violence similar to that which brought the country close to civil war in May 2008.

The 2008 unrest that left around 100 people dead was the culmination of an 18-month political crisis that erupted in 2006 after Hezbollah withdrew its ministers from the government