The Israel Defense Forces (IDF) on Friday said it had bombed several residential buildings in southern Beirut, claiming Hezbollah's "central headquarters" were embedded underneath.
Massive explosions could be heard across Lebanon's capital about 6:20pm local time (1:20am Saturday AEST). Large plumes of smoke were also visible.
Previous IDF strikes in Beirut have been attempts to kill the leadership of the Hezbollah militant group.
Senior Hezbollah commanders were the target of Israel's strike, a senior Israeli official said, adding it was too early to tell whether the attack killed Hezbollah leader Hassan Nasrallah.
In a statement released hours after the bombing, Hezbollah said nothing about its general secretary's condition. It said there was "not truth to any statement" about Israel's strikes.
It also denied that any weapons or arms depots were located in buildings that were hit in an Israeli strike on Beirut suburbs, the group's media office said in a statement.
Around two hours after the strikes, Lebanon's health ministry said six people had been killed and 76 had been wounded.
While Israel has increased its attacks on Lebanon significantly this week, it's believed Friday evening's strikes were the largest since the conflict broke out.
Witnesses said the strikes sent shockwaves across the city, as people rushed off the streets.
Residents in social media groups started posting videos shot by residents living near the location of the bombing showing large explosions.
Hezbollah's al-Manar television network claimed four buildings had been destroyed.
The bombing marks another escalation with the Iran-backed militant group.
In a video statement shortly after the blasts, IDF spokesperson Rear Admiral Daniel Hagari confirmed the attack.
He said Hezbollah's headquarters had been "intentionally built under residential buildings" and that the militant group "must be stopped".
Residents in Beirut were shaken by a series of loud strikes, which initially sounded like a sonic boom Israel frequently creates over the Lebanese capital.
"When the second sound came, we realised it couldn’t be sonic booms," Celine Ghattas told Reuters reporters on the ground.
"The sounds kept coming one after another. Of course, we got scared.
"People started running out of the church, and we were terrified. We said that (Israel) started striking here and there, without targeting specific individuals."
The attack came about half an hour after Israeli Prime Minister Benjamin Netanyahu addressed the United Nations General Assembly in New York, where he said there was "no place" Israel's weapons could not reach in the Middle East.
He subsequently cut short his trip to New York and returned to Israel.
While Israel has exchanged fire with the Iran-backed Hezbollah for almost 12 months, leading to thousands of people evacuating on both sides of the border, it has escalated its attacks this week.
Hezbollah began shelling northern Israel the day after the October 7 terrorist attacks by Hamas across southern Israel, which killed 1,200 Israelis.
The Lebanese health ministry said about 700 people had been killed since Monday and thousands more injured, although that was before Friday evening's largest bombardment.
Earlier this week, the United States and France attempted to broker a deal that would have seen a pause in hostilities between Israel and Hezbollah. The proposal was snubbed by Mr Netanyahu.
Lebanon's Prime Minister Najib Mikati said the latest strikes showed that Israel did not care about a ceasefire.
US President Joe Biden said his country had no prior knowledge of Israel's planned strikes.
