The leader returned to Beirut following claims Saudi officials had ordered his resignation – an allegation both Riyadh and Mr Hariri deny.
In his resignation speech, Hariri had cited fear of assassination, and attacked Iran along with Hezbollah for sowing strife in the Arab world.
But Lebanon’s President Michel Aoun refused to accept his withdrawal from office because it took place in ”mysterious circumstances” abroad.
President Aoun had previously claimed Prime Minister Hariri had been kidnapped by Riyadh.
Hundreds of Hariri supporters packed into the streets near his home, where the leader told them he would “stay with them to be a line of defence for Lebanon, Lebanon’s stability and Lebanon’s Arabism”.
Supporter Manar Akoum, 26, said: ”His presence in the country alone brings stability.”
The US welcomed the Lebanese Prime Minister’s return, with a State department spokesperson confirming they were encouraged by Mr Hariri’s discussions with President Aoun and his statement reaffirming his commitment to the stability of Lebanon.
The development comes amid fears Saudi Arabia and Iran are on the brink of war as tensions in the Middle East reach breaking point.
The regions of Qatar, Yemen, Syria and Lebanon are also embroiled in the bitter conflict.
Lebanon was thrown into the centre of the crisis as the country is home to the Iranian-backed terrorist militia Hezbollah – who claimed Saudi Arabia was holding Mr Hariri hostage.
Relations between the Sunni-majority Saudi Arabia and the Shia-dominated Iran have deteriorated recently due to the ongoing civil war in Yemen and political crisis in Lebanon.
Tensions increased between the two countries earlier this month when Saudi Arabia accused Iran of being behind a ballistic missile attack on a Riyadh airport.
The missiles were intercepted and Saudi Arabia said they perceived the attack as a “declaration of war”.
Saudi Arabia’s Foreign Minister Adel Al-Jubeir said: “The kingdom will not stand by and will not hesitate to defend its security.
“We must stand together”.
The regional director of the Middle East and Africa at the Economist Intelligence Unit, Pat Thakar, told CNBC that it would not take much for tensions in the region to flare up.
The expert said if war were to break out between Saudi Arabia and Iran “it would be the most dangerous time for the Middle East and the world”.
She said: “But neither Saudi Arabia nor Iran want war.
“Saudi Arabia will fight tooth and nail to retain that position as the head of Islam and the Middle East.
“It will even go to bed with Israel to stop Iran.”