Saad al-Hariri resigned, using a television interview broadcast from Saudi Arabia to declare he fears an assassination plot while also criticising Iran.
He accused Iran of spreading “fear and destruction” across several countries, including Lebanon.
Mr Hariri said: “We are living in a climate similar to the atmosphere that prevailed before the assassination of martyr Rafik al-Hariri.
“I have sensed what is being plotted covertly to target my life.”
However, Iran-backed Hezbollah accused Saudi Arabia of forcing Mr Hariri to quit, after his resignation toppled a coalition government that inlaced Hezbollah, thrusting Lebanon back into the frontline of the Saudi-Iran rivalry.
Sayyid Hassan Nasrallah, Hezbollah’s leader, said: “The resignation was a Saudi decision dictated to Prime Minister Saad al-Hariri and was forced on him.”
He added Lebanon’s political leaders expect Mr Hariri to return on Thursday if he is allowed to travel by Saudi Arabia.
Lebanese President Michel Aoun will not accept Mr Hariri’s resignation until he returns to the country to explain his actions, according to sources.
Sunni Muslim Saudi Arabia, an ally of Mr Hariri, is locked in a scrap for power across the Middle East with Shi’ite Iran, and its allies, which include Hezbollah.
Saudi Arabia accused Iran of an “act of war” after a missile was intercepted over capital city Riyadh.
The long-range ballistic missile was fired by Houthi rebels in Yemen and intercepted by a major airport near Riyadh.
However, in a dramatic turn, the Saudi-led coalition claimed the missile was an act of aggression by Iran, who reportedly smuggled weapons to the Yemeni militias, according to Zero Hedge.
A statement, broadcast by state media, said: “The Coalition’s Command considered this a blatant act of military aggression by the Iranian regime, and could rise to be considered as an act of war against the Kingdom of Saudi Arabia.”