Hezbollah chief Hassan Nasrallah has warned that the militant group is ready for "open war" with Israel, after the killing of one of its leaders in Syria.
Nasrallah blamed Israel for Imad Mughniyeh's assassination
Nasrallah made the declaration during a fiery speech at the funeral of Imad Mughniyeh in Lebanon's capital, Beirut.
There is huge tension in the city, where thousands are attending Mughniyeh's funeral and rivals have held a memorial for ex-PM Rafik Hariri.
It is three years since Hariri was killed, plunging Lebanon into crisis.
Mughniyeh was killed in a car bombing in the Syrian capital, Damascus, on Tuesday.
Correspondents say the events come at a tense time, with no president and no working parliament.
A huge security operation is under way amid fears of clashes between the pro-Syrian Hezbollah supporters and the anti-Syrian Hariri supporters.
About 8,000 army and internal security force troops have been deployed, with the aim of keeping the two factions apart.
The BBC's Mike Sergeant in Beirut says the two events will separate them naturally, but there are fears that people departing from mixed neighbourhoods could clash with rival groups on the streets.
Israel on alert
Mughniyeh's funeral, which Iranian Foreign Minister Manouchehr Mottaki is attending, is taking place in the Hezbollah stronghold of the southern suburbs of Beirut.
Speaking to the crowds of mourners via a giant video screen, Nasrallah blamed Israel for his death and said its war against the Jewish state was not over.
"Zionists, if you want this sort of open war, then let the whole world hear, so be it!" he declared.
Nasrallah said that Israel thought that Mughniyeh's death "would lead to the destruction of the resistance... but they are wrong".
He insisted: "The blood of Imad Mughniyeh will make them [Israel] withdraw from existence."
Israel has rejected Hezbollah's claims that it is responsible for Mughniyeh's assassination, but nonetheless has put its embassies and other missions around the world on high alert and boosted troop deployments on the Lebanese border.
Security sources said the alert could remain in force for weeks or even months.
Call for unity
Lebanon has been experiencing some of its worst internal violence since the assassination of Mr Hariri in a Beirut car bombing sparked a political crisis three years ago.
His murder led to massive domestic and international pressure which forced Syria to withdraw its troops from Lebanon after a presence of 29 years.
Three years on, thousands of Lebanese braved the pouring rain to gather on Martyrs' Square in Beirut's city centre where Mr Hariri is buried.
Many carried flags and photos of Mr Hariri and other anti-Syrian politicians killed in recent years in attacks.
To symbolise the unity of Lebanon's religious communities, church bells rang out at the same time as the Muslim call to prayer.
Lebanese Culture Minister Tarek Mitri made an appeal for national unity.
"The hazard of history has made it such that a large number of Lebanese will be commemorating the loss of Prime Minister Hariri while others will be mourning, a large number of Lebanese too, would be mourning one of the leaders of Hezbollah," he said.
"Let us hope that this will not be a divisive factor, another divisive factor, in Lebanon. We need to heal our divisions, we need to transcend what divides us."
But our correspondent in Beirut says that in recent days, the political rhetoric has been getting sharper on all sides and has now been joined by heightened emotions following the killing of Mughniyeh.
'Murderer and terrorist'
Mughniyeh, in his late 40s, had been variously described as special operations or intelligence chief of Hezbollah's secretive military wing, the Islamic Resistance.
He was top of the US "most wanted" list until he was replaced by Osama Bin Laden and other al-Qaeda leaders following the 11 September 2001 attacks.
Analysts say his death is a significant blow to Hezbollah, which battled Israel in the 2006 Lebanon war with help from its Iranian and Syrian backers.
Hezbollah was founded in 1982 by a group of Shia Muslim clerics after the Israeli invasion of Lebanon. It has emerged in recent years as a major political and military force in Lebanon, after military successes against Israel.