Saad al-Hariri, the son of Rafiq al-Hariri, who was assassinated in February 2005, said on Wednesday: "I call on everybody who loves Rafiq al-Hariri, everybody who wants the truth, the international tribunal and an end to the assassinations ... to come out with us tomorrow."
Rafiq al-Hariri, a former prime minister, was killed by a truck bomb in Beirut along with 22 other people.
Pierre Gemayel was shot dead while driving through a Beirut suburb on Tuesday.Leaders call for protests
They are among several prominent anti-Syrian politicians to have been murdered in the last two years.
Walid Jumblatt, the leader of the Druze sect and Samir Geagea, head of the Lebanese Forces, also called on the Lebanese to attend the funeral of Gemayel - a Maronite Christian and the son of Amin Gemayel, a former president.
"It seems the Syrian regime will continue with the assassinations," Jumblatt said on Wednesday.
"It seems the Syrian regime will continue with the assassinations"
Walid Jumblatt, Druze leader
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Some leaders say the assassins want to weaken a government opposed to Damascus's influence in Lebanon and which took power after the withdrawal of its troops.
The government has already been undermined by the resignation of six Shia ministers in recent weeks.
If two more ministers resign, the government will fall.
Syria has denied playing any role in the deaths of any of the 16 anti-Syrian figures who have died violently in the past two years.
UN to investigate murder
The United Nations Security Council has also approved a request from Fouad Siniora, the Lebanese prime minister, for UN help in investigating the assassination of Gemayel.
On Wednesday, the 15-nation council unanimously ordered the UN commission investigating the murder of al-Hariri to help the Lebanese authorities track Gemayel's killer.
The UN inquiry led by Serge Brammertz, a Belgian, is already looking into 14 other apparently politically motivated attacks in Lebanon since al-Hariri's killing.
Brammertz has reported that all 15 cases are linked in some way.
Gemayel's slaying would bring the total number of cases under UN investigation in Lebanon to 16.