The walkout ushers in the country's worst political crisis since 2008 in one of the most volatile corners of the Middle East.

Prime Minister Saad Hariri, the son of the slain leader, cut short a visit to Washington after meeting with President Barack Obama. He was heading to Paris where he will meet French President Nicolas Sarkozy on Thursday, his office in Beirut said.

Hariri planned to hold consultations on his government's collapse while in France, then would return to Beirut, according to an official in Hariri's delegation who spoke on condition of anonymity to discuss sensitive diplomatic moves.

U.S. Secretary of State Hillary Rodham Clinton said the Obama administration was "consulting closely with concerned parties and nations as to the best way forward to preserve the sovereignty, stability and independence of Lebanon and the needs of the Lebanese people."

"We view what happened today as a transparent effort ... to subvert justice and to undermine Lebanon's' sovereignty and independence," she told reporters in Doha, Qatar.