The leader of Lebanon's Iranian-backed Hezbollah group has declared "mission accomplished" after scoring major gains in parliamentary elections, as the main Western-backed faction headed by Prime Minister Saad al-Hariri lost a third of its seats.

Shia group Hezbollah and its political allies gained at least 43 seats, giving another boost to Iran's allies in Lebanon and Syria, where Tehran's influence has grown in recent years as it has provided crucial support to Syrian President Bashar al-Assad.

The results also underline the growing clout of the group in Lebanon, where it dominates politically and militarily.

Hezbollah supports the Syrian Government of Bashar al-Assad, and is known for its confrontations with Israel.

Mr Hariri acknowledged the losses at a news conference in Beirut on Monday (local time), but said "it's not the end of the world".

Sorry, this video has expired Lebanon's Prime Minister acknowledges loss of seats in election

The international community, he said, should look at the results in a "positive way" because they reflected democracy in Lebanon.

"My hand is extended to every Lebanese who participated in the elections to preserve stability and create jobs," Mr Hariri said in a televised statement.

Mr Hariri, a Sunni politician with close ties to Saudi Arabia, said he would continue to work closely with President Michel Aoun, who is allied with the rival, Hezbollah-led bloc.

He told reporters that his Future Movement won 21 seats in Sunday's vote, a decline of 11 from the last election, in 2009.

Mr Hariri would still have the largest Sunni bloc in Parliament, facilitating his return as prime minister to form the next government.

Hezbollah declares 'major' victory

Hezbollah leader Hassan Nasrallah said his party's gains would give "protection" to the group and declared "mission accomplished" after weeks of campaigning.

Hezbollah is considered a terrorist group by the US, but its political wing has long held seats in Lebanon's Parliament and was part of Lebanon's outgoing coalition government.

Mr Nasrallah did not say how many seats his group and its allies won, but the official results showed they took at least 43 of the legislature's 128 seats, giving them the power to veto laws.

"There is a major political, parliamentarian and moral victory for the choice of the resistance," Mr Nasrallah said in a televised speech.

Hezbollah supporters took to the streets to celebrate. ( Reuters: Aziz Taher )

Hezbollah supporters rode through the streets of Beirut on scooters, honking their horns and waving the militant group's signature yellow flag as some shouted sectarian slogans.

The election, the first in nine years, was marked by a low turnout, especially in the capital, Beirut.

Interior Minister Nouhad Machnouk put national turnout at 49 per cent, compared to 54 per cent in 2009. In Beirut precincts, the turnout was between 32 per cent and 42 per cent.

Iran welcomed the initial election results, with Foreign Ministry spokesman Bahram Ghasemi saying that his country would "support and cooperate" with any government that was elected by a majority.

The next Cabinet, like the outgoing one, will likely be a unity government that includes Hezbollah. The two sides can hardly govern effectively without each other, something Mr Hariri alluded to at his news conference.

"The country needs stability … Lebanon needs jobs, development, tourism and electricity," he said.

AP