An Israeli minister said the outcome, which has yet to be confirmed by official results, showed the Lebanese state was indistinguishable from Hezbollah, signalling the risk of Israel hitting Lebanon's government in a future war.
Many of Hariri's traditional supporters appear to have stayed at home on Sunday for the first parliamentary vote in nine years.
It has one of the world's highest debt-to-GDP ratios and the International Monetary Fund has warned its fiscal trajectory is unsustainable.
Hezbollah is proscribed as a terrorist group by the United States and its hold on Lebanese affairs has been problematic for a succession of USA and European leaders.
"This is a great political and moral victory for the resistance", Nasrallah said, in reference to his party's identity as bulwark against Israeli and Western aggression.
Rival blocs in parliament could not agree on a new president between 2014-16 and repeatedly chose to delay elections, partly because of disagreement over moving from a winner-takes-all to a proportional voting system.
"The vote results provide the Mustaqbal with a bloc of 21 lawmakers in the parliament". The pro-Hezbollah al-Akhbar newspaper called it "the slap" for Hariri on its front page.
A coalition constituting Hezbollah and allied parties - including Amal, another Shiite party, and the Free Patriotic Movement, the country's largest Christian party - is poised to capture a majority of the seats in Lebanon's parliament according to preliminary results reported on Monday.
Hezbollah, along with affiliated groups and individuals, secured at least 67 seats, according to a Reuters calculation based on preliminary results for almost all the seats obtained from politicians and campaigns and reported in Lebanese media. As he specified, "the parliamentary presence" created by Hezbollah and its allies would guarantee the protection of the "resistance".
"This will make the strength of Hezbollah at the very end".
According to Associated Press, Hariri lost five seats in Beirut where his party had a strong hold once.
Lebanon's politics has been divided by the Syrian war, with some parties supporting the Iran-sponsored Hezbollah's intervention in Syria to aid President Bashar Assad, and other Saudi-aligned parties opposed to it. He is the scion of a famous political dynasty and son of late former prime minister Omar Karame, who was an ally of Damascus. He blamed a new electoral law and a performance "that wasn't up to the standard".
His remarks at a televised address came after early results recounted Hezbollah's landslide victory in the election.
The anti-Hezbollah Christian party Lebanese Forces also appears to have done well in the election, with indications that they have nearly doubled their MPs from eight to 15.
Senior political leaders, including Hariri himself and Interior Minister Nohad Machnouk whose task it was to supervise the vote, admitted that new voting rules tested for the first time had been perplexing for the electorate. Bassil fell short in Lebanon's last elections in 2009.
As for the so-called candidates for civil society: They have only won a few seats that will allow them to improve women's representation in the parliament, raising their number to seven.
In order to win in the Lebanese capital, however, Hariri needed a 50% voter turnout in Beirut, which clearly did not happen on Sunday. Two of the 10 seats there were won by its opponents, one going to the Lebanese Forces and the other to Future.
Gabriel Debbane, general manager of Debbane Saikali Family Holding, told Xinhua at a Sidon polling station that he was not satisfied with the way that things went in the past years, especially the power outages and garbage issue.
Turnout for the polls was disappointing. Within the nine-year period in between, the nation's parliament extended its mandate three times under the pretext of political instability.
This did not translate into widespread enthusiasm for the electoral process, however, with voter turnout standing at just under 50 per cent.
Interior Minister Nohad Machnouk announced the turnout figure at a news conference shortly after midnight and appeared to blame it on the new electoral law agreed past year. Nasrallah said that he could not predict developments in the confrontation with Israel, but is confident that "we can't lose time" after winning the election.
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It would also be nice to see a more competitive side to the sport, which usually sees one side dominate the entire season. The team is looking at Charles Leclerc from Ferrari, Marcus Ericsson or Antonio Giovinazzi who is a reserve for Ferrari.