By Tom Perry and Oliver Holmes
BEIRUT, Aug 8 (Reuters) - Former Lebanese prime minister Saad al-Hariri returned home on Friday for the first time in three years, on a visit seen as reasserting his leadership over the Sunni community following a deadly incursion by Islamist militants in northeast Lebanon.
Hariri, Lebanon's most influential Sunni politician, has been in self-imposed exile since 2011, sharing his time between France and Saudi Arabia. He left Lebanon after his government was toppled by a coalition including the Iranian-backed Shi'ite group Hezbollah.
With no prior announcement, Hariri arrived at the Lebanese government's headquarters in Beirut, where he met Prime Minister Tammam Salam.
The Saudi-backed politician arrived in a Mercedes with blacked-out windows at the central courtyard of the Grand Serail and grinned widely as he walked into the building.
Hariri earlier this week announced that Saudi Arabia would donate $1 billion in military aid to Lebanese security forces to help them in the fight against extremists.
"My return comes after the Saudi donation which requires seeing how it can be implemented and translated into support for the army," Hariri said on his Twitter account.
The Twitter account also said Hariri's first stop would be at the grave of his father, Rafik al-Hariri, another former Lebanese prime minister whose assassination in 2005 forced Saad to enter political life.
Hariri blames Syrian President Bashar al-Assad for the bomb attack in Beirut that killed his father. A special tribunal in the Netherlands has been trying four members of Hezbollah in absentia for the killing.
The group, an ally of Assad, denies any involvement.
Hariri's visit follows a deadly incursion by Islamist militants who crossed from Syria and seized the Sunni town of Arsal in the northeast last Saturday. The gunmen withdrew from the town on Wednesday after five days of battles with the army.
The incursion by militants, including fighters affiliated to Islamic State which has seized large areas of Iraq and Syria, marked the most serious spillover to date of the three-year-old Syrian conflict.
Assad cracked down on Syria's pro-democracy movement in 2011 in a move that has led to a full-scale civil war pitting Sunni rebels against Assad's Alawite sect and Shi'ite fighters which has also had ramifications for tiny neighboring Lebanon.
Rocket fire, suicide attacks and gun battles connected to Syria's war have plagued Lebanon and the conflict has worsened the perennial political deadlock in the Mediterranean country, with officials divided largely along sectarian lines.
The deadlock has left Lebanon without a president since May, when incumbent Michel Suleiman's term expired.
The coastal city of Tripoli has seen regular skirmishes between Sunni and Alawite militiamen. Firebrand Sunni clerics such as Salafist leader Ahmad al-Assir have rallied Sunnis to fight the Beirut government, which includes Hezbollah members.
"There has been, in the last three years, a vacuum that has formed in the Sunni community. This was becoming increasingly dangerous because this community was becoming more and more radicalized," said Michael Young, a political commentator.
"(Hariri's) return is probably an effort with the Saudis to reassert a certain amount of control over the Sunni community." (Additional reporting by Alexander Dziadosz; Editing by Gareth Jones)