First anniversary of Harriri's death
Hariri's death in a truck bombing on 14 February 2005 prompted huge street protests and the withdrawal of Syrian forces stationed in Lebanon.
Lebanon's President Emile Lahoud said on Monday that his government and he himself are determined to reveal the truth of the assassination.
Top Syrian officials have been implicated in Hariri's murder, and President Bashar Assad was entangled in a confrontation with the United States and the West, which have pressed him to cooperate more actively with the UN investigation into the explosion. Damascus' relations with Beirut have been at a low ebb in decades.
Crowds are expected to gather on Tuesday near the shrine to Hariri at a square in central Beirut - also the scene of a massive anti-Syrian protest in March last year. Al-Hariri's son Saad has returned from a self-imposed six-month exile in Saudi Arabia for the occasion. Supporters of Hezbollah and members of Christian leader Michel Aoun's political faction are not expected to participate.