Preliminary results suggested Hariri's Sunni, Druze and Christian alliance would emerge from Sunday's poll with 71 of the assembly's 128 seats, against 68 before, after close electoral battles in Christian heartland districts.
The outcome, a blow to Iran and Syria, will reassure the United States, which had warned it would review aid to Lebanon depending on the shape and policies of the next cabinet.
Whether the election will lead to another uneasy national unity government or plunge Lebanon into prolonged political deadlock may hinge on the outside powers who back the opposing blocs, wedded to irreconcilable visions for the country.
It may take weeks to agree on a new government, but few analysts expect political disputes to spark armed confrontation.
"Lebanon will not witness another round of violence," said Hilal Khashan, political science professor at the American University of Beirut. "There's a regional understanding on this, which we saw at work yesterday when despite the heated atmosphere, the election took place peacefully."
But he forecast "rough days ahead" while political adversaries clash over the formation of the next government.
For a year, a Qatari-brokered deal backed by regional rivals Syria and Saudi Arabia has checked tensions in Lebanon, allowing state institutions to function again after an 18-month impasse that ended in a spasm of street fighting in May 2008.
US President Barack Obama's diplomatic overtures to Iran and Syria, Hezbollah's principal patrons, have also helped contain regional hostilities that often spill over in Lebanon.
The Lebanese have no stomach for a repetition of last year's echo of their 1975-90 civil war, analyst Rami Khouri said.
"They realise that what they did last May, the Sunni-Shi'ite fighting in the streets, was a catastrophe. They want to avoid that at any cost," he said of the clashes in which Hezbollah and its allies briefly took over the Muslim half of Beirut.
Ultimately, another inclusive, power-sharing government was inevitable, he argued. "The real election was last May in the streets and Hezbollah won. That defined the power structure that came out of Doha and that is going to continue -- nobody is going to force decisions down the throat of the other side."
Mutual Mistrust
But trust between the two blocs is in short supply.
Hezbollah's tough Shi'ite guerrillas, who fought a 34-day war with Israel in 2006, are viewed as a liability, if not a dangerous Iranian proxy, by Hariri's Western-oriented bloc.
Suspicions run just as high among the "resistance" camp led by Hezbollah, hostile to what it sees as U.S.-inspired efforts to deprive it of the arms needed to defend Lebanon from Israel.
Post-election bickering may focus on whether Hezbollah, its Shi'ite ally Amal and Aoun's loyalists should again be granted veto power over key decisions in a consensus government.
Hezbollah MP Hassan Fadlallah called for "partnership" -- a code word for veto rights -- and argued against any repeat of past "catastrophes" that proved no one could monopolise power.
"Whoever wants political stability, preservation of national unity and the resurrection of Lebanon will find no choice but to accept the principle of consensus," he told Reuters.
Druze leader Walid Jumblatt, a prominent figure in Hariri's bloc, told Reuters after the election that Hezbollah and its allies should join the next cabinet, but without veto power.
"We should not forget that the election should be a boost to the dialogue and we should not try to isolate the other parties," he said, referring to talks outside parliament among Lebanese leaders focused on the fate of Hezbollah's arms.
Jumblatt attributed his side's victory to its stance that "we should incorporate, slowly but surely, the weapons of Hezbollah inside the Lebanese army and that the decision of war and peace should be taken only by the Lebanese state".
It will take more than a narrow election result in Hariri's favour to advance that vision while the Middle East remains polarised, with Iran, Syria, Hezbollah and the Palestinian Hamas group ranged against Washington, its Arab allies and Israel.
But Obama's new tone, along with his demand that Israel halt settlement building in the occupied West Bank, may begin to blur the divide widened by the "war on terror" his predecessor George W. Bush declared after the Sept. 11 attacks on U.S. cities.
The next test of that will be Iran's presidential election on Friday, when President Mahmoud Ahmadinejad will be trying to fend off rivals who advocate less confrontation with the West.
"Iran's impact on Lebanon is a derivative of the U.S.-Iran relationship," Khouri said. "If they move toward more talks and a softer tone, you will see that reflect positively in Lebanon." (Additional reporting by Tom Perry and Laila Bassam)
