Celebratory gunfire erupted near Hariri's headquarters in Beirut and jubilant supporters took to the streets of several cities as the results started filtering out from the election, whose outcome is being closely watched by Lebanon's neighbours and the international community.
Police and soldiers were out in force in sensitive areas for fear of fighting between rivals in a war-scarred country that remains deeply divided along sectarian lines.
Analysts said the winning factions would have to work with their rivals to form a government and ensure the nation is not plunged into a new cycle of political instability and violence.
Hasan Fadlallah of Hezbollah refused to acknowledge defeat for the Iranian- and Syrian-backed alliance -- whose emergence as a possible victor in the election had caused jitters in the West.
"What matters to us now is that Lebanon turns a new page, one based on partnership, cooperation and understanding," he told AFP. "No party can claim to have won the majority among all communities."
Official results are expected to be known later on Monday.
Voters turned out in force for what was considered one of Lebanon's most crucial elections. Preliminary estimates put turnout at more than 54 percent of the 3.2 million electorate, the highest since at least the end of the 1975-91 civil war.
Analysts had predicted that just a handful of seats were likely to separate the rival blocs in the battle for control of parliament.
The International Crisis Group said in a report last week that the vote was likely to revive rather than resolve underlying conflicts and that forming a viable government would "require difficult compromise on all sides."
Under Lebanon's complex power-sharing system, the seats are divided equally between majority Muslims and minority Christians, who make up about a third of the four-million population.
Hezbollah itself fielded just 11 candidates but heads an alliance grouping the Shiite Amal movement and the Free Patriotic Movement, a nationalist party headed by Christian civil war army chief Michel Aoun.
It is expected that the final results will show that Christian vote, which was divided between the rival camps, proved crucial.
Former US president Jimmy Carter, heading a team of international observers, said he hoped Lebanon's political parties and their foreign backers would accept the results.
"We don't have any worries over the conduct of the elections," Carter said. "We have concerns over the acceptance of the results by all the major parties."
During polling day, there were no reports of serious problems, although three people were arrested for using fake identity cards and the army intervened in one city after some voters traded insults and blows.
International observers said most problems seemed to have caused by the high turnout, with some voters complaining of a long wait to cast their ballot.
Israel, which fought a devastating war with Hezbollah guerrillas in 2006, had warned that victory for the Shiite group would pose a danger to the entire region.
The United States also blacklists Hezbollah as a terrorist organisation and had warned that continued military aid to Lebanon would hinge on the vote outcome.
The current Sunni-led majority swept to power in 2005 on a wave of popular anger following the assassination of Rafiq Hariri in a massive Beirut car bombing.
The murder was widely blamed on Syria, which denied any involvement, and the ensuing public outcry led to Damascus withdrawing its troops from Lebanon after a 29-year presence.