The poll on Sunday is the fourth of a series staggered by region over four weekends. It is the first in Lebanon with no Syrian military presence in three decades after Damascus pulled its troops out in April.
The northern Lebanon vote, in which more than 100 candidates are fighting for the remaining 28 parliamentary seats, will decide whether the 128-seat assembly has an anti-Syrian majority for the first time since the 1975-1990 civil war.
Competition is close and tension high.
The anti-Syrian list backed by the son of slain prime minister Rafiq al-Hariri squares off against an unlikely alliance between pro-Syrian figures and Damascus's sworn enemy, former general Michel Aoun.
Caretaker Prime Minister Najib Miqati described turnout as "good" as he cast his vote in the main northern city of Tripoli.
The al-Hariri-owned Future Television estimated that 20% of voters had cast their ballots four hours after polls opened, putting turnout on track to top the 40% recorded in 2000 by the official close at 6pm (1500 GMT).
Security was tight for the election in a region where Syrian troops long held sway, but Miqati rejected al-Hariri bloc accusations that voters had been intimidated by a continuing Syrian intelligence presence.
Prime Minister Najib Miqati said
voting was completely free
"The voting has been completely free," insisted Miqati, who was approved as a compromise figurehead by the opposition despite his links with Damascus. "There's been no intervention by Syria to influence voters. There are no Syrian agents at work."
European and other foreign observers were monitoring the conduct of the vote which the international community had demanded be held on time after the turmoil caused by al-Hariri's killing.
Polls in the north
Aoun's sweeping victory in the Christian heartland of Mount Lebanon in last week's round stunned the disparate movement, whose street protests, following al-Hariri's 14 February assassination, forced Syria to bow to global pressure and pull out of Lebanon.
Saad al-Hariri's (L) group must
win 21 seats for a full majority
Saad al-Hariri's slate must now win 21 of the seats up for grabs in the north to scrape an absolute majority in the house - a far cry from the commanding two-thirds the anti-Syrian front had predicted would give it more weight in the cabinet.
If al-Hariri's coalition, which swept the polls in Beirut three weeks ago, fails to secure those seats, it will become just one of three substantial blocs in the assembly, forced to bargain with anti-Syrian rivals and compromise with allies of Damascus.
"The election taking place in the north ... is among the most important in the history of Lebanon and represents a unique opportunity to change for the better," al-Hariri said last week.
Whoever fares better, unlikely alliances that characterised the election are likely to crumble once the results are out. They will realign into three main blocs - the anti-Syrian faction led by al-Hariri, the pro-Syrian group dominated by the Shia Muslim Amal and Hizb Allah and Aoun's followers.
They will jostle for a say on divisive issues such as President Emile Lahoud's fate and international calls for Hizb Allah to disarm.
A debate has raged throughout the campaign over whether pro-Syrian Lahoud, a centre of controversy since Syria pressed parliament to extend his term last year, should stay or go.
The bloc, led by al-Hariri and Druze leader Walid Jumblatt, hold Lahoud at least indirectly responsible for the elder al-Hariri's death and want him out. A UN investigation into the killing, demanded by that bloc, began this week.
But if it falls short of a parliamentary majority, it will face an uphill struggle getting rid of the president and replacing him with one of its own.
Aoun has become the undisputed
leader of Lebanon's Christians
Aoun's victory has identified him as the undisputed leader of Lebanon's Christians, making it harder to put forward other names for president, a position traditionally reserved for a Maronite Christian.
"This is a decisive battle and is a continuation of last week's battle in the mountains," Aoun told Reuters on Saturday.
Seats in the house are divided equally between Christians and Muslims - who include Shia, Sunnis and the Druze sect. Muslims represent about 53% of voters.