Mr Hariri also attacked the Iran-backed Shia movement Hezbollah, which wields considerable power in Lebanon. "To the extent that Lebanon works and functions, Hezbollah derives legitimacy from being part of that", said Brandon Friedman, a scholar at Tel Aviv University's Dayan Center. This was quickly judged to be more than a rhetorical flourish by Sabhan, as the Saudi minister stressed that Riyadh will henceforth treat Lebanon as a "hostile government".
"What happened was Saudi madness and we'll wait and see what happens over the next few days", Nasrallah told his associates.
It was not immediately clear who might succeed Hariri, Lebanon's most influential Sunni politician.
The prime minister must be a Sunni in Lebanon's sectarian system.
Mr Hariri's father, former prime minister Rafik al-Hariri, was assassinated in 2005. A U.N. -backed tribunal has charged five Hezbollah members over the killing.
Hariri had served as a prime minister between 2009 and 2011 as well.
Aoun's office said Hariri had called him from "outside Lebanon" to inform him of his resignation.
Direct military action would risk huge destabilization in the Gulf and beyond, disrupting oil shipments vital to Saudi Arabia and its allies.
Hariri flew to Saudi Arabia on Friday after a meeting in Beirut with Ali Akbar Velayati, the top adviser to Iranian Supreme Leader Ayatollah Ali Khamenei. In 2015, a coalition of Saudi Arabia, the UAE and other allies began a campaign against the Shiite rebels known as Houthis, who they say are tools of Iran.
Still, the kingdom is trying to hold influence in the mainly Sunni areas that remain out of Assad's hands.
"The Saudis want to create a situation of political crisis, to break the political stability in Lebanon so that Hezbollah will be more busy in Lebanese internal politics than in Syria", he said.
Sabhan said Hariri resigned unexpectedly on Saturday during a trip to Saudi Arabia because his life was in danger. Iran's military is larger and more battle-hardened than Saudi Arabia's, but the kingdom has far more advanced weaponry bought from the U.S. and Europe in the past decade. In recent weeks, leading Christian politicians who oppose Hezbollah have also visited Saudi Arabia. But it has succeeded in killing thousands of innocent people, displacing millions, and helping Tehran position itself as the defender of the oppressed in the Middle East. Central Bank Governor Riad Salameh sought to calm fears the political turmoil would hit Lebanon's already fragile economy, issuing a statement to reaffirm the stability of its currency, which is pegged against the US dollar.