The Hezbollah-led opposition has been calling for the government of Fuad Siniora, the current prime minister, to step down.
Leaders from both sides have called for calm after the bombings on the two buses, which also injured at least 18 people, in a predominantly Christain area northeast of the capital.
Saad al-Hariri's coalition has blamed Tuesday's bombings on Syria, which is also accused of being behind the Febuary 14, 2005 suicide truck bombing that killed Rafiq al-Hariri and 22 other people.
Syria has repeatedly denied involvement in the assassination, which triggered international pressure that forced Damascus to withdraw thousands of troops from Lebanon in 2005, ending years of Syrian domination.
Al-Hariri said he wanted the anniversary to be "an occasion for the Lebanese to emphasise their adherence to the truth" and call for an international court to try suspects in the killing.
"Not going out is bending to fear, giving in to what they want to do to us," he told LBC television.
The UN Security Council and the Lebanese government have approved plans for the court but Lebanon's pro-Syrian president has not ratified the deal and Nabih Berri, the parliament speaker and leader of the opposition Amal party, has refused to call the chamber to vote on the plan.
Opposition leaders dispute the legitimacy of the government and say they support the idea of the tribunal but want to discuss its mandate to make sure it is not used against them.
Hassan Nasrallah, the Hezbollah leader, has apologised for his decision not to attend Wednesday's commemoration.
In an article in As-Safir newspaper, he said, addressing the late Rafiq al-Hariri, "our only sin is that we refused to accuse without evidence or to turn your blood into a weapon for vengeance".