Syrian intelligence agents have pulled out of their Lebanese headquarters in the border town of Anjar, reports say.
Lebanese troops moved into Anjar after Syrian troops pulled out
Rustom Ghazaleh, Syria's intelligence chief in Lebanon, was also seen heading towards the Syrian capital, Damascus.
He is expected to return to Lebanon on Tuesday for a farewell ceremony for the last remaining Syrian forces.
Meanwhile, the head of the Lebanese security service, Maj Gen Jamil Sayeed, says he has resigned because of the changing political scene.
His resignation - which he offered last week along with the head of internal security - meets a main demand of the anti-Syrian opposition.
Witnesses said they saw Syria's intelligence agents and troops withdraw from Anjar in the Bekaa Valley on Monday.
Mr Ghazaleh's office was left empty and a Lebanese flag was seen flying over the building.
Tuesday's ceremony will mark the formal end of Syria's 29-year military and intelligence involvement in Lebanon.
A United Nations team will be dispatched to Beirut to confirm the pullout has been completed - four days ahead of a 30 April deadline set by Damascus.
SYRIA IN LEBANON
Military intervention began in 1976
30,000 troops in Lebanon during 1980s, currently 14,000
Syrian forces helped end Lebanese civil war in 1990 and maintain peace
Calls for Syrian withdrawal increased in 2000 after Israeli pull-out from southern Lebanon
UN resolution calling for foreign forces' withdrawal in Sept 2004
But correspondents say that verifying the end of Syria's influence over Lebanese politics will prove more difficult.
Lebanon's security services have come in for heavy criticism for their handling of the investigation into the killing of former Prime Minister Rafik Hariri on 14 February.
Opposition politicians and many members of the public believe Syria was behind the assassination, and that the Lebanese security services, which have tended to come under Syria's sway, were somehow involved.
Syria denies any involvement, and security service officials deny any wrongdoing.
"Security chiefs are usually appointed with politics and change when it changes," Mr Sayyed - who was seen as pro-Syrian - said in his resignation letter.
Correspondents say Mr Sayyed is widely seen as Lebanon's most powerful security figure since the end of the civil war in 1990.