Hariri said that he feared being assassinated and blamed the Iranian-backed Lebanese militant group Hizbollah.
It comes as Hizbollah fighters take control of more land in Syria alongside the Syrian army.
Hizbollah joined the Syrian civil war and helped dictator Bashar Al-Assad crush the Syrian revolution.
Describing Hizbollah as “Iran’s arm”, Hariri said the group had “managed to impose a fait accompli on Lebanon through the power of its weapons”.
He said, “Iran is trying to destroy the Arab world, and Hizbollah’s weapons are aimed at Syrians and Lebanese.”
Yet there are strong indications that Hariri was forced to resign by another major regional power— Saudi Arabia, Iran’s biggest rival in the Gulf.
Saudi Arabia has been fighting a bloody proxy war against Iranian-backed Houthi rebels in Yemen—killing thousands of Yemeni civilians in airstrikes.
Just last week Saudi Arabia, which is backed by the US and heavily armed by Britain, killed 21 people in an airstrike on a crowded market.
Houthi rebels fired a missile at Riyadh shortly after Hariri’s resignation.
Hariri has close ties to Saudi Arabia, and he made his resignation speech from the Saudi Arabian capital Riyadh rather than Lebanese capital Beirut.
Just one year after becoming prime minister in November 2016, Hariri had shown no sign of resigning until his speech. And the Lebanese army said it had not uncovered any plots to assassinate him.
Shortly after Hariri’s speech Saudi Arabia’s crown prince Mohammed bin Salman had almost 50 Saudi princes, ministers and businessmen arrested for “corruption”.
The moves are seen as part of Prince Mohammed’s attempts to strengthen his power. He is also aligning Saudi Arabia closer to Israel against their common enemy Iran.
Israel’s government is also worried about Iran’s growing control in Syria, and is preparing for conflict with Hizbollah.
Israeli airstrikes against Syrian regime forces have further ratcheted up tensions. The Syrian army responded to an airstrike last week by firing a missile at the Israeli fighter plane.
It shows the competition to carve up the Middle East amid the ruins of Syria and Iraq will almost certainly mean more bloodshed.