Lebanese Prime Minister Saad Hariri announced his resignation on Saturday, citing Iran's "grip" on the country and threats to his life.
The surprise move risks plunging the small and already fragile Middle Eastern country deeper into turmoil.
"I announce my resignation from the post of prime minister," Hariri said in a speech broadcast from Saudi Arabia by the Al-Arabiya news network.
"I felt what was being covertly plotted to target my life," he said.
Hariri's personal security concerns appeared to gain little traction among the public. Social media were flooded with messages deriding him for choosing to resign from abroad and on a foreign channel.
The two-time premier, whose father Rafik held the same position for years and was assassinated in 2005, accused Iran and its powerful Lebanese Shiite ally Hezbollah of seeking hegemony in the region.
The 47-year-old Sunni politician's resignation comes less than a year after his government, to which Hezbollah's political wing belongs, was formed.
"Iran has a grip on the fate of the region's countries... Hezbollah is Iran's arm not just in Lebanon but in other Arab countries too," Hariri said.
He accused Tehran of "sowing discord among the children of the same nation and creating a state within the state... to the extent that it gets the final say on how Lebanon's affairs are run".
Iran dismissed his accusations.
Hariri's "repetition of unreal and baseless accusations... against Iran show that the resignation is designed to create tensions in Lebanon and in the region", said Iran's foreign ministry spokesman Bahram Ghassemi.
Hariri also lambasted Hezbollah.
"In recent years, Hezbollah has used the power of its weapons to impose a fait accompli," he said, reading his speech from behind a desk.
Hezbollah is a vital ally of Syrian President Bashar al-Assad in the war the Damascus regime is waging against the Islamic State group and armed opposition movements.
It enjoys broad support from Iran and is the only Lebanese party to have kept its weapons after the 1975-1990 civil war.
Hezbollah's arsenal has since grown exponentially and now outstrips that of the nation's own armed forces.
The group claims it is the only credible rampart against neighbouring Israel, and its refusal to disarm is the main political crux in Lebanon.
Israeli Prime Minister Benjamin Netanyahu said Hariri's resignation and his accusations against Iran should serve as a "wake-up call" to the world about Tehran's ambitions.
Hezbollah members have been accused over the 2005 assassination in a massive car bomb blast of Rafik Hariri, the dominant figure in Lebanon's post-war political landscape.
He made his fortune in Saudi Arabia, where his son Saad was born.
Saudi Arabia is Iran's main regional rival, and the two powers' tussle for influence has played out in ongoing conflicts in Iraq, Syria and Yemen.
The office of Lebanese President Michel Aoun, a veteran Christian leader allied to Hezbollah, issued a statement saying it was waiting for Hariri's return to Lebanon "to enquire about the circumstances of his decision and decide on the next steps".
Hariri said in his speech that the political climate in Lebanon was reminiscent of that which prevailed before his father was killed.
Risks of war
The February 2005 assassination triggered political upheaval that led to Syria's military withdrawal from Lebanon.
Walid Jumblatt, one of Lebanon's political heavyweights and the country's most prominent Druze leader, said Hariri's resignation could adversely affect a country already under huge strain.
He argued it was the latest manifestation of the tug-of-war between Saudi Arabia and Iran, and called for intensifying diplomatic efforts to solve the feud.
"Lebanon is too small and vulnerable to bear the economic and political burden that comes with this resignation," he said on social media. "I will continue to call for dialogue between Saudi Arabia and Iran."
Even as he resigned, Hariri warned his foes: "Our nation will rise just as it did before and the hands that want to harm it will be cut."
Lebanese political analyst Hilal Khashan argued that Saudi Arabia had been piling the pressure on its protege lately and "summoned" him to Riyadh.
He said Hariri's move could start "a cold war in Lebanon that could escalate into a civil war" or even a regional offensive on Hezbollah.
It is unclear who could replace Hariri at this stage.
Under a power-sharing system that helped end Lebanon's 15-year civil war, the president must be a Christian, the prime minister a Sunni and the speaker a Shiite.