President Lahoud wants to appoint an army chief as his successor
However, mediators fear rival camps will fail to reach a deal, plunging the country into a deeper political crisis.
Repeated attempts to elect a new president over the past two months have been scuppered by rivalry between Western-backed and pro-Syrian factions.
French Foreign Minister Bernard Kouchner, who has been trying to broker a solution, said despite complications "a miracle is still possible".
But his Italian counterpart, Massimo D'Alema, was pessimistic on the eve of Friday's deadline.
"Tomorrow, I don't believe there will be an election and this will create difficult conditions," he said.
The French, Italian and Spanish foreign ministers have spent several days in Lebanon, meeting rival groups in an attempt to break the deadlock.
A vote in parliament has been scheduled for 1300 (1100 GMT), 11 hours before current President Emile Lahoud's term expires.
The election of a president requires a two-thirds majority, which means that the anti-Syrian ruling bloc - with its slim majority - cannot force its preferred candidate through parliament. A deal with the opposition is therefore required.
The rival factions cannot agree on a compromise candidate, however.
And the opposition has warned it may boycott Friday's session, thus ensuring the quorum will not be reached and any vote will be invalid.
According to Article 62 of the Lebanese constitution, if no candidate is elected before Mr Lahoud's mandate expires, his powers are automatically transferred to the anti-Syrian government of Prime Minister Fouad Siniora.
However, President Lahoud has vowed not to hand over power to Mr Siniora, and said he would name army chief General Michel Suleiman as his provisional successor instead.
Opposition leader Michel Aoun proposed a compromise on Thursday, whereby an interim president would be selected to fill the office until parliamentary elections were held in 2009.
This was dismissed by the ruling majority, however, which said the plan was unconstitutional.
The political deadlock has already led to the vote being postponed four times since 25 September.
The BBC's Kim Ghattas in Beirut says the failure to find a compromise has raised fears of civil strife, including the possibility that the opposition could create a rival administration, as happened during the civil war.
Our correspondent says the issue is turning into a regional and international affair.
The US, Russia, Syria and Iran are all intensely involved and there has been a lot of diplomatic shuttling between Damascus, Moscow, Tehran and Paris.
US Secretary of State Condoleezza Rice telephoned several of the country's top leaders on Monday.
Fears are growing that failure to elect a president will bring more turmoil to Lebanon.
The army has increased its presence on the streets of Beirut and set up checkpoints, some schools have cancelled classes until Monday, and the ministry of interior has suspended all firearm permits until further notice.