His group, the Future Movement, saw its parliamentary share shrink by about one-third, to 21 seats from 33 seats, in the 128-member body, Mr. Hariri told reporters on Monday.
In Lebanon’s complex, sect-based political system, the prime minister must be a Sunni, while the speaker of Parliament is a Shiite and the president is a Maronite Christian.
While Mr. Hariri’s movement lost ground to rival Sunnis, he still appeared poised to keep his job as prime minister.
It remained unclear late Monday when the Lebanese government would release the official results for the poll, which was held on Sunday. That would begin negotiations to form a government, a process that could drag on for weeks or months.
Lebanon’s unique sectarian make up and place in the region make its politics about local issues like jobs, infrastructure and garbage collection as well as about regional rivalries and alliances. In general, the country’s politics have long broken down between a camp affiliated with Iran and one oriented toward Saudi Arabia and the United States.
Mr. Hariri, the leader of the latter camp, appears to have lost ground because his supporters were disappointed by his performance and because of the concessions he had to make to rival parties to secure his post as prime minister, said Emile Hokayem, an analyst at the International Institute for Strategic Studies, who was in Lebanon for the vote.
The strong showing by Hezbollah and its allies could jeopardize the country’s regional and international standing at a time when its leaders are counting on international support to prop up the economy, support the military and deal with the burden of nearly 1 million refugees from neighboring Syria.
“Lebanon is very exposed to the Iran-centered turmoil in the region, so Hezbollah and the others are going to have to navigate carefully if they want to avoid the consequences of that,” Mr. Hokayem said.
Hezbollah’s leader, Hassan Nasrallah, said in a televised speech on Monday that the results would give “protection” to what his supporters call “the resistance,” or the regional alliance opposed to Israeli and American influence.
Kassem Qassir, a Lebanese political analyst close to Hezbollah, said that the elections represented a victory for “the Iranian axis” but that Hezbollah’s leaders would seek to make decisions that were good for the country as a whole.
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“They don’t want to control Lebanon,” Mr. Qassir said. “The country is facing many challenges, and Hezbollah is in need of international and outside support.”
Other regional dynamics affected the vote.
Mr. Hariri has long been close to Saudi Arabia, although that relationship has been under strain since a bizarre episode last year when he was summoned to the Saudi capital, Riyadh, and forced to announce his resignation.
While Mr. Hariri appears to have mended his ties to the kingdom and its powerful crown prince, Mohammed bin Salman, the Saudis did not provide generous financial support for his campaign as they have in the past.
In recent years, Saudi Arabia and Israel have become increasingly alarmed about Iran’s growing influence in the Arab world, and they are likely to see further evidence of this in the election results.
On Monday, a member of Israel’s security cabinet said that the Lebanese state had become indistinguishable from Hezbollah, which he said would change Israel’s calculus should it wage a new war against the militant group.
“The State of Israel will not differentiate between the sovereign State of Lebanon and Hezbollah, and will view Lebanon as responsible for any action from within its territory,” Naftali Bennett, the education minister in Israel’s conservative coalition government, wrote on Twitter on Monday.
The Lebanese government reported voter turnout at 49 percent, down from 54 percent the last time legislative elections were held in 2009. In Beirut, home to about half of the country’s estimated 4.5 million people, turnout was between 32 and 42 percent, depending on the district.
While returning many of Lebanon’s power brokers to office, the elections also brought in new faces. An unprecedented number of women candidates ran, and the number in Parliament appeared to rise to seven from four, which is low compared to other states in the region.
The campaign also saw the participation of dozens of independent candidates who said they represented “civil society” and who focused on services like water and electricity and criticized the established parties for corruption and their inability to address the country’s pressing problems.
At least one was reported to have won, a female journalist.
Also winning a seat was Jamil al-Sayyed, a retired general and former intelligence chief who is a close friend of President Bashar al-Assad of Syria.