Lebanon's year-old unity government collapsed on Wednesday after Hezbollah ministers and their allies from the so-called March 14 alliance resigned over tensions stemming from a UN-backed tribunal investigating the 2005 assassination of former Prime Minister Rafik Hariri.
The walkout ushers in the country's worst political crisis since 2008 in one of the most volatile corners of the Middle East.
The tribunal is widely expected to name members of Hezbollah in upcoming indictments, which many fear could re-ignite sectarian tensions that have plagued the tiny country for decades.
The resignations were announced at a news conference by Energy Minister Jibran Bassil, a member of the Free Patriotic Movement - the key Christian ally of Shiite Hezbollah.
Hezbollah, which is backed by Iran and Syria, has denounced the tribunal as an "Israeli project" and urged Western-backed Prime Minister Saad Hariri - the son of the slain politician - to reject any findings by the court even before it announced any indictments.
But the prime minister has refused to break cooperation with the tribunal.
Hariri formed the current national unity government in November 2009, but it has struggled to function amid deep divisions.
Hariri himself was set to meet on Wednesday with President Barack Obama in Washington to discuss the crisis in Lebanon.
His office meanwhile, had no immediate comment.
The walkout followed the failure of a diplomatic push by Syria and Saudi Arabia to ease political tensions in Lebanon.
There had been few details about the direction of the Syrian-Saudi initiative, but the talks were lauded as a potential Arab breakthrough, rather than a solution offered by Western powers.
Labour minister Butros Harb said later on Wednesday: "This situation puts us into administrative crisis and also in a new political crisis which increases complications in the country and does not contribute towards solving any of the problems."
The crisis over the tribunal has paralysed the government in recent months.
Violence has been a major concern as tensions rise in Lebanon, where Shiites, Sunnis and Christians each make up about a third of the country's four (m) million people.
In 2008, sectarian clashes killed 81 people and nearly plunged Lebanon into another civil war.
Rafik Hariri's assassination in a suicide bombing that killed 22 other people both stunned and polarised the Lebanese people.
He was a Sunni who was a hero to his own community and backed by many Christians who sympathised with his efforts in the last few months of his life to reduce Syrian influence in the country.
A string of assassinations of anti-Syrian politicians and public figures followed, which UN investigators have said may have been connected to the Hariri killing.
The Netherlands-based tribunal has not said who it will indict, but Hezbollah leader Sheik Hassan Nasrallah has said he has information that members of his group will be named.
