Hundreds of Beirut residents fled their homes late on Sunday after Israel said it was preparing attacks.
Soon after the warning, several blasts were heard and a large fire was seen in Beirut's southern suburbs, Reuters has reported.
There was no immediate information on what caused the explosion, or details of any casualties.
Israel had said it was going to focus its attacks on sites linked to the financial operations of Lebanon's Hezbollah group within hours and told people to leave those areas immediately.
Panicked crowds clogged the streets and caused traffic jams in some parts of Beirut as they tried to get to neighbourhoods thought to be safer, witnesses said.
"Residents of Lebanon, the IDF (Israeli military) will begin attacking infrastructure belonging to the Hezbollah Al-Qard Al-Hassan Association - get away from it immediately," the military's spokesperson said in a statement on X.
Al-Qard al-Hassan - which the US has said is used by Iran-backed Hezbollah to manage its finances - has more than 30 branches across Lebanon including 15 in densely populated parts of central Beirut and its suburbs.
There was no immediate statement from the organisation, Hezbollah or the Lebanese government.
Sunday's warning of attacks came hours after Israel said it hit Hezbollah's intelligence headquarters in Beirut.
In the northern Gaza Strip, officials said rescuers were still recovering people from the rubble after an Israeli attack on Beit Lahiya that left 87 people dead or missing on Saturday, according to the health ministry - one of the highest death tolls for months from a single attack.
Israel said it was investigating reports of the incident.
The attacks and the warnings of more marked an intensification of Israel's offensives both in Gaza and Lebanon, days after the killing of Hamas leader Yahya Sinwar raised hopes of an opening for ceasefire negotiations to end more than a year of conflict.
Prime Minister Benjamin Netanyahu said he was the subject of an assassination attempt by "Iran's proxy, Hezbollah" on Saturday when a drone was directed at his holiday home.
In a call with former US president Donald Trump, the prime minister reiterated that Israel would make decisions based on its own interests, according to a statement from Mr Netanyahu's office.
