The group is known as the "March 8" alliance.
The US, which lists Hezbollah as a "terrorist" group, has linked future aid to Lebanon to the shape and policies of the government that replaces the current national unity cabinet.
The West and other countries, including Saudi Arabia, back the ruling "March 14" alliance, which swept to power in 2005, following the murder of Rafiq al-Hariri, a former prime minister, in a massive car bombing in Beirut.
It is led by Saad al-Hariri, Rafiq al-Hariri's son.
A win for Hezbollah and its Maronite Christian allies could precipitate a tilt towards Syria and would likely increase regional tensions.
Gil Hoffman, the chief political correspondent for Israel's Jerusalem Post
, told Al Jazeera he believed Hezbollah would use an election win to attack Israel.
"The worst case scenario is of course that Hezbollah wins," he said.
"Hezbollah has been amassing three times as many missiles as they had before the war they started a couple of years ago, and they would immediately use the leadership of Lebanon to attack Israel," he said, referring to the war fought in 2006 between Israel and Hezbollah in Lebanon.
Israel has been clear that, should Hezbollah win, it will treat Lebanon as an enemy state.
Joseph Gebeily, a spokesman for the Lebanese Forces Party, a part of the March 14 coalition, warned that Lebanon "would enter into a regional confrontation if Hezbollah is leading a coalition".
"It would be a nightmare for Lebanon," he told Al Jazeera.
Ali Hamdan, a spokesman from Amal, a party allied to Hezbollah, told Al Jazeera that the international focus on the elections had taken the emphasis away from social issues.
"By God's grace, if it happens that [we win], we will make serious improvements," he said.
"The way that people - the international community - people who have an interest in Lebanon are trying to raise up the issue as something else ... that's why you don't see the Lebanese internal problems."
The elections look likely to be decided largely by Lebanon's divided Christian districts.
The Free Patriotic Movement's Aoun, allied to Hezbollah, faces Christian rivals in the shape of the Phalange party led by Amin Gemayel, a former president, and the Lebanese Forces led by Samir Geagea, both members of the March 14 camp.
Al Jazeera's James Bays, reporting from Lebanon, said the Christians appeared set to emerge as the election "kingmakers".
"To put it simply, the Christians are bitterly divided. The Christians find themselves in the two different camps - there are Christian parties on both sides of the political divide," he said.
"They are the fluid voters that might go either way. So the Christians are going to end up - even though they're very divided - as the kingmakers."
Although the West backs the March 14 coalitions, Western nations have not ruled out dialogue with a Hezbollah-led government.
Jimmy Carter, the former US president, who is acting as an election observer in the Lebanese vote, suggested the US administration would engage with the Lebanese government led by Hezbollah.
"I was talking last night to the White House representatives," he told reporters.
"They reminded me that in [US] President [Barack] Obama's speech he made it very clear that the United Sates would accept the results of elections around the world from a democratic society, if they are honest and fair and free."
In the run-up to the elections, much of the campaigning has focused on Hezbollah's military power, which is stronger than the state's security forces.
|The build up to Lebanon's elections has
largely been calm [Reuters]
Opponents say Hezbollah's weapons undermine the state, while the group and its allies see them as crucial to defending Lebanon from Israel.
Sectarian tension brought Lebanon to the brink of civil war last year when more than 100 people were killed in violence before an agreement led to the election of Michel Sleiman, then the army chief, as president and the formation of a national unity government.
Lebanon's power-sharing system divides the 128 seats in parliament equally between Christians and Muslims.
The country's president must be a Maronite Christian, the prime minister a Sunni Muslim and the speaker of parliament a Shia Muslim.
Polls are due to close at 7pm local time (16:00 GMT).