A Lebanese election official empties a ballot box after the polling station closed during Lebanon's parliamentary election, in Beirut, Lebanon, May 6, 2018.
Hariri said his group won 21 seats in Sunday's vote, 11 fewer than what it had been holding since 2009.
"Hariri's loss will be the distinguishing mark of these elections, which will have consequences on the battle to form a new government", the pro-Hezbollah Al-Akhbar daily wrote earlier on Monday. Economic stagnation and the general disappointment over the devastating war in neighbouring Syria leading to a massive influx of refugees into Lebanon might have propelled them against the prime minister, the AP report said.
The powerful Shia group is a key political player in Lebanon where it has allied with the Christian party of President Michel Aoun and has participated in Hariri's government since December 2016.
The Iran-backed Shiite movement and its allies look set to secure a parliament bloc large enough to thwart attempts for it to disarm, a longstanding demand of its political enemies. Both Hariri and the Hezbollah leader, Hassan Nasrallah, were expected to speak later in the day. "Hezbollah will benefit from the lack of a broad coalition against it", political analyst Imad Salamey said.
Hezbollah has sent thousands of its fighters to Syria to shore up regime leader Bashar al Assad's forces. The United States designates Hezbollah as a terrorist group and also arms and trains the Lebanese army.
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Turnout was 49.2 percent, down from 54 percent the last time legislative elections were held nine years ago.
In Beirut precincts, the turnout was between 32 percent and 42 percent. He blamed a complex new voting law and gaps in his party's performance.
Some voters also said that the sometimes absurd web of local alliances that saw some parties work together in one district and compete in others had put them off.
Hezbollah as well as groups and individuals affiliated to it have won at least 67 seats in Lebanon's parliament, according to the results cited by politicians and campaigns and reported in Lebanese media.
Hezbollah's seat gain may complicate relations with Western countries.
Branded a terrorist group by the U.S., the heavily armed Shia Hezbollah has grown in strength since joining the war in Syria in support of President Bashar al-Assad in 2012.