More than a dozen explosions rocked Beirut, in the most intensive strikes on the southern suburbs since a war between Israel and Hezbollah in 2024.
People fled on foot and by car, clogging the roads, after the series of strikes began in the early hours of Monday.
The violence widened the conflict that has spread through the Middle East since the United States and Israel attacked Iran on Saturday.
Hezbollah, a Shi'ite Muslim group established by Iran's Revolutionary Guards in 1982, is one of Tehran's principal allies in the Middle East.
The Israeli military said it had begun striking Hezbollah targets across Lebanon, including senior Hezbollah members in the Beirut area.
Lebanese security sources said the airstrikes hit several areas of the southern suburbs, known as Dahiyeh.
"Hezbollah opened a campaign against Israel overnight, and is fully responsible for any escalation," Israeli Chief of the General Staff, Eyal Zamir, said in a statement.
The Israeli military issued a warning ordering residents of dozens of villages in southern and eastern Lebanon to evacuate.
The Lebanese health ministry could not immediately be reached for a casualty toll.
The Israeli military said several projectiles that crossed from Lebanon fell in open areas and one was intercepted by the Israeli airforce.
"No injuries or damages were reported," it said.
Hezbollah said it had targeted an Israeli military missile defence facility south of the city of Haifa in revenge for "the pure blood" of Khamenei and in response to what it described as repeated Israeli attacks.
It was the first time the group has carried out such an attack since the 2024 war.
"The resistance leadership has always emphasised that the continuation of Israeli attacks and the assassination of our leaders, youth, and people gives us the right to defend ourselves and respond at the appropriate time and place," Hezbollah said in a statement.
Since a US-backed ceasefire between Israel and Lebanon in 2024, Israel has carried out regular strikes against what it has identified as Hezbollah targets in Lebanon, accusing the group of seeking to rearm.
It was Israel's first attack on the southern suburbs since it killed the group's top military official, Ali Tabtabai, in November.
Israel also carried out airstrikes in southern Lebanon and the Bekaa Valley in eastern Lebanon, the Lebanese security sources said.
"The IDF (Israel Defense Forces) will operate against Hezbollah's decision to join the campaign, and will not enable the organisation to constitute a threat to the State of Israel," the Israeli military said.
Israel and Lebanon agreed to a US-brokered ceasefire in 2024, ending more than a year of fighting between Israel and Lebanese militant group Hezbollah that had culminated in Israeli strikes that severely weakened the Iran-backed group.
Lebanese Prime Minister Nawaf Salam said the firing of projectiles from south Lebanon was irresponsible, calling it a suspicious act that jeopardised Lebanon's security.
Lebanon's presidency said on Saturday it had been told by the US ambassador that Israel would not escalate against Lebanon as long as there were no hostile acts from the Lebanese side.
Hezbollah has fought numerous conflicts with Israel, but was severely weakened by Israel in a war in 2024 when its leader Hassan Nasrallah was killed.
with AP
