The surprise return home by Hariri, the country's most prominent Sunni leader, comes after he announced on Wednesday that Saudi King Abdullah had promised him Riyadh would provide Lebanon with USD 1 billion to fight jihadists on the Syrian border.
Hariri's trip comes at a key moment for Lebanese Sunnis, who comprise about a third of the population of a country where Sunnis generally support the rebellion against President Bashar al-Assad in neighbouring Syria.
Hariri, an early supporter of moderate rebels in Syria, opposes extremists who have become stronger in Syria and also have a presence in Lebanon, mainly in the northern port of Tripoli, Sidon in the south and the Bekaa valley in the east.
On Saturday, clashes erupted for the first time between jihadists from Syria and Lebanese forces in Arsal, a pro-rebel Sunni stronghold near the border with Syria.
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Hariri, 44, has constantly voiced his unconditional support for the Lebanese army, calling it a "red line" not to be crossed.
Hariri left the country in 2011 a few months after his government collapsed following the resignation of ministers from Hezbollah, his main political rival.