He was killed by a massive car bomb in the capital, Beirut, along with 21 other people.
Mr Hariri's death led to mass protests against Syria which eventually withdrew its troops from Lebanon.
However an international tribunal indicted five members of Lebanon's Shia movement, Hezbollah, for the attack.
Hezbollah denied it was responsible, rejecting the charges as political, and the defendants remain at large.
Thousands of people are expected to gather for a memorial at the site of Hariri's grave in Martyrs Square in Beirut - where religious leaders gathered to pay their respects on Friday.
Several senior politicians, including the veteran Druze leader Walid Jumblatt, laid wreaths at the late premier's grave on Saturday.
A speech is due to be given by Hariri's son - former Prime Minister Saad Hariri - who arrived in the country overnight.
"Lebanon lost an exceptional leader who had upheld the country on the global map and regained its distinctive place in the heart of the Arab world, after years of a doomed [civil] war," said Prime Minister Tammam Salam.
"We salute the memory of the man who dedicated his life entirely to achieving Lebanon's security, sovereignty, pride and prosperity."
Lebanon has been through many upheavals - but if a single event stands out as a clear turning point, it was Hariri's shocking death in an explosion that was clearly the product of a complex conspiracy involving powerful quarters, says the BBC's Jim Muir in Beirut.
He had fallen out sharply with the Syrians, so many thousands of Lebanese instinctively blamed Damascus - and following huge demonstrations they pulled their forces out, ending a military presence lasting nearly 30 years.
Lebanon is now desperately vulnerable to the fallout from a civil war raging in Syria, our correspondent adds - far worse than anything Lebanon itself produced.
Ceremonies and prayers of remembrance are being held in Lebanon on the 10th anniversary of the assassination of the former Prime Minister Rafik Hariri.