Thousands of people from rival camps are taking part in key memorials amid heightened tension in Beirut.
Supporters of Hezbollah are attending the funeral of Imad Mughniyeh, a senior member of the militant group.
Speaking to mourners, Hezbollah leader Hassan Nasrallah blamed Israel for his death, and said its war against the Jewish state was not over.
Meanwhile anti-Syrian groups marked three years since the assassination of former PM Rafik Hariri.
A huge security operation is under way in the city amid fears of clashes between pro- and anti-Syrian factions.
Correspondents say the events come at a potentially explosive time, with no president and no working parliament.
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.
The country has been experiencing some of its worst internal violence since the assassination of Mr Hariri plunged Lebanon into crisis three years ago.
His murder sparked 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 have gathered on Martyrs' Square in Beirut's city centre where Mr Hariri is buried.
They are standing in the pouring rain with flags and photos of Mr Hariri and other anti-Syrian politicians killed in recent years in attacks.
"Open our parliament, free our government, elect a president now," reads one banner.
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 appealed 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 says in recent days the political rhetoric has been getting sharper on all sides - add into the mix heightened emotions following the killing of Mughniyeh on Tuesday in a car bombing in the Syrian capital, Damascus.
His funeral is taking place in the Hezbollah stronghold in the southern suburbs of Beirut.
Iranian Foreign Minister Manouchehr Mottaki is attending, and Hezbollah leader Hassan Nasrallah addressed the crowd on a giant video screen.
The militant group urged supporters to walk behind Mughniyeh's coffin, describing him as a "leader of whose leadership we should be proud".
"Let us make our voice heard by all the enemies and murderers that we will be victorious, no matter the sacrifices," a Hezbollah statement read out on TV said.
'Murderer and terrorist'
In a separate development, Israel put its embassies and other missions around the world on high alert and boosted troop deployments on the Lebanese border in response to Mughniyeh's death.
Security sources said the alert could remain in force for weeks or even months.
Hezbollah and Iran have blamed Israel for his killing, but it has rejected the accusations.
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 had been 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 will be 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.