Lebanese opposition group Hezbollah and its allies say they will boycott a parliamentary session called for Tuesday to elect a new president.
Pro-Syrian opposition members say they will stay away from parliament if there is no consensus with the anti-Syrian ruling majority on a presidential candidate.
Lebanese parliament speaker and opposition leader Nabih Berri called the session to pick a successor to his Syrian-backed ally, President Emile Lahoud, whose term ends in November 24.
An opposition boycott of parliament would prevent it from mustering a two-thirds quorum required for a vote. Lebanon's president is chosen by parliament, rather than the public.
Berri said Monday he is optimistic a deal can be reached with the Western-backed government on a new president by November.
Lebanon's anti-Syrian ruling coalition holds a narrow majority in parliament, which was cut further by last week's assassination of Christian legislator Antoine Ghanem in a Beirut car-bombing.
Several anti-Syrian Christian candidates are running for the Lebanese presidency against Christian leader Michel Aoun, who is backed by the pro-Syrian opposition. Only Maronite Christians can vie for the post under Lebanon's sectarian power-sharing system.