Tens of thousands of black-clad mourners vowed support for Hezbollah at the Beirut funeral of slain leader Hassan Nasrallah on Sunday, nearly five months after he was killed in an Israeli airstrike.

Nasrallah shared the funeral with his cousin and successor, Hashem Safieddine, who was killed in an Israeli airstrike on a Beirut suburb a few days later.

Since Saturday, roads into Beirut have been clogged with carloads of supporters travelling in from Hezbollah's other power centres in south and east Lebanon.

"When I saw the coffin, reality dawned upon me," said Lara, 26, adding that she had a hard time coming to terms with his killing.

"The pain is great... words cannot describe how I feel," she added.

A founding member of Hezbollah in 1982, Nasrallah won renown around the Arab world in May 2000 when Israel ended its 22-year occupation of south Lebanon following relentless attacks by the group under his leadership.

In the decades since, Lebanese have been divided over Hezbollah, with many criticising the group for initiating more recent hostilities with Israel in support of Palestinian militant group Hamas.

As the funeral began, Israeli warplanes flew at a low altitude over Beirut in what Defence Minister Israel Katz said was a "clear message" to anyone who threatens Israel.

"You will specialise in funerals - and we in victories," Katz said.

In a televised address at the ceremony, Nasrallah's successor Naim Qassem said Hezbollah would keep following his "path", and rejected any control by the "tyrant America" over Lebanon.

"The resistance is not over, the resistance is still present and ready" to face Israel, he said.

Israel has carried out multiple strikes in Lebanon since a November 27 ceasefire deal with Hezbollah ended more than a year of hostilities, including two months of all-out war. (Agencies)