|Lebanese Christian leader Michel Aoun casts his ballot in a polling station in Haret Hreik|
Voters in Bekaa Valley and Mount Lebanon, the most populous regions of Lebanon, are casting their ballots.
Fifty-eight of the 128 seats in parliament will be decided in this round. Because so many seats are stake, it is likely that the winner of this round will control Lebanon's parliament for the next four years.
These elections mark the first time in 30 years that Lebanese have voted without the presence of troops from neighboring Syria.
Following the assassination of Prime Minister Rafiq Hariri in February, Syria was pressured to pull out its army. His killing was blamed on Damascus, which has denied any role in the killing.
But while Syria has withdrawn its troops, its presence is felt in these elections. In the two earlier rounds, anti-Syrian candidates won the first and pro-Syrian candidates won the second. Both results were as expected.
This round is considered much harder to call. One of the most keenly watched contests pits allies of Christian leader Michel Aoun against a ticket led by anti-Syrian Druze leader Walid Jumblatt.
General Michel Aoun had been expected to create alliances with other anti-Syrian opposition members but is now heading his own breakaway faction, which is running against other candidates opposed to Syria. A spokesman for General Aoun, Ziad Abas says the fierce competition should prompt a high voter turnout.
"There will be a big turnout because everybody feels involved," he said. "This is the only district where there is really a political battle between two viewpoints in contrast to all other electoral districts, especially the ones that happened before in Beirut and the south where there was not really a campaign or a battle."
The final round of parliamentary elections will take place in the north of Lebanon on June 19.