The collapse of Lebanon's government has plunged the country into deep political uncertainty as the president begins putting a new administration together.
President Michel Suleiman asked Saad Hariri to stay on as caretaker prime minister after the Iranian-backed militant group Hezbollah and its allies resigned on Wednesday and brought down Mr Hariri's government.
The crisis was the climax of tensions that have been simmering for months over the UN tribunal investigating the 2005 assassination of Mr Hariri's father, former prime minister Rafik Hariri.
The tribunal is widely expected to indict members of Hezbollah soon, which many fear could rekindle violence in the tiny nation plagued for decades by war and civil strife.
Lebanon suffered through a devastating civil war from 1975-1990, a 1982 Israeli invasion to drive out Palestinian fighters in the south followed by a 20-year occupation of the south, a 2006 war between Israel and Hezbollah, and deadly sectarian fighting between the country's Sunnis and Shiites in 2008.
The collapse of the government ushers in the worst political crisis since 2008 in one of the most volatile corners of the Middle East.
Arab League chief Amr Moussa expressed concern that Lebanon could again descend into chaos.
"It is bad. It is tense. It is threatening," he said of the situation. "All of us have to work together in order to reach some kind of compromise," he told reporters in Doha, Qatar.
Israel also said it was worried about renewed violence on its northern border with Lebanon. Israeli troops in the north of the country are on alert.
Lebanon's 14-month-old unity government was an uneasy coalition linking bitter rivals - a Western-backed bloc led by Mr Hariri and the Shiite Hezbollah - in an attempt to stabilise the country. But in reality, it had been paralysed for months because of disputes over the Hariri tribunal.