Formed as a resistance movement during the Israeli occupation of Lebanon in the early 1980s, Hezbollah is today a political, military and social organisation that wields considerable power in the country.
Hezbollah has also sent thousands of its fighters to Syria to support forces loyal to President Bashar al-Assad in battles against predominantly Sunni rebel forces and the jihadist group Islamic State (IS). But Hariri will be further constrained in what he can do by Hezbollah, said analysts, which will complicate Western policy toward Lebanon, a recipient of major USA and European aid.
A day after Lebanon's first general election in nine years, Hezbollah and its allies look set to secure a bloc large enough to block attempts for it to disarm, a longstanding demand of its political enemies.
In less than two years, the movement has expanded into a coalition of 11 civil society groups and independents competing for seats under one banner called Kollouna Watani (Arabic for We Are All the Nation).
The new, pre-printed ballots used on Sunday perplexed some voters, causing delays in polling stations.
In Zahle, politician Mryiam Skaff accused members of the right-wing Lebanese Forces party of beating up her supporters in polling stations.
The polling stations across the country opened doors at 7 a.m. local time (0400GMT) and were closed at 7 p.m. local time (1600GMT). In the district of Choueifat south of Beirut, a crowd rebelled against a station supervisor and smashed a ballot box. The army ordered the media to turn off their cameras.
Unofficial results show the resistance Movement Hezbollah and its allies have managed to gain more than half of the votes.
Hassan Nasrallah said in a televised speech Monday that the "mission is accomplished" after weeks of campaigning. That, and its intervention in Iraq and Yemen, has led several oil-rich Gulf states to also name it as a terrorist group.
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"We were betting on a better result", the premier said.
"The parliamentary bloc that Hezbollah will be able to control will not be confined to the 27 Shia MPs, it will be constituted of the 27 Shia MPs and their allies in other communities", he said. "If you want, I can tell you that we have won", Hariri said at a press conference in Beirut.
He said now there are a lot of problems in the chaotic Middle East, so the peaceful Lebanese election sets a good example for achieving new changes and reconciliation not in military ways, but in political ways.
Mr Hariri said his party had ended up with 21 seats, down from 33 nine years ago.
The main party leaders will start in the coming days to negotiate the shape of a new government, but analysts said there likely will be no change with Hariri remaining as prime minister and Aoun staying on as president.
He says he will continue to work closely with President Michel Aoun, who is allied with a rival bloc led by the militant Hezbollah.
Lebanese Prime Minister Saad al-Hariri said on Monday that the new electoral law allowed parties to commit election violations. Analysts predict that influential Shiite movement Hezbollah, backed by Iran and wielding a formidable arsenal it refused to give up after the civil war, would retain or slightly increase its clout in the legislature.
Hariri, a Sunni politician with close ties to Saudi Arabia, has so far lost five seats in Beirut, once considered a stronghold for his party.
Hezbollah and its allies appear set to take at least 47 seats in the 128-seat parliament, enabling them to veto any laws the group opposes.