Hassan Nasrallah has declared "victory" for his Hezbollah movement against self-declared jihadists in Syria as tens of thousands of Shia Muslims gathered in Beirut to commemorate Ashoura.
The Shia group has sent thousands of fighters into neighbouring Syria to fight alongside the troops of President Bashar al-Assad.
We want to win the final victory... so that the region does not fall into the hands of beheaders... and rapists
Addressing supporters in Beirut's southern suburbs by video link on Tuesday, Nasrallah said that Sunni radicals, known as takfiris, "have no future".
"These takfiris [Muslims who accuse other Muslims of apostasy] will be defeated in all areas and countries, and we will feel honoured that we played a role in their defeat," he said.
The Ashoura commemorations took place amid tight security because of threats linked to Syria-related violence in Lebanon.
"Security measures are the most extreme that they have ever been," Al Jazeera's Stefanie Dekker, reporting from the capital Beirut, said.
"These are sensitive times for Lebanon and, of course, this has to do with the spillover from the war in Syria."
The commemorations mark the killing of Imam Hussein, the grandson of the Prophet Mohammed, who was killed at the hands of soldiers of the caliph Yazid in 680 AD, an event that lies at the heart of Islam's sectarian divide into Shis and Sunni sects. Some Sunnis also mark Ashoura.
Beirut's southern suburbs - a mainly Hezbollah stronghold - have seen a string of deadly attacks, many of them claimed by self-declared jihadist groups, since Hezbollah started sending fighters to Syria three years ago.
"We are now in the fourth year [of the Syrian war], and the extremists have failed to take over Syria ... it's a great victory," Nasrallah said.
"We want to win the final victory ... so that the region does not fall into the hands of beheaders ... and rapists," he said, referring to the Islamic State in Iraq and the Levant group, which has carried out widespread atrocities in Syria and Iraq.
The Syrian conflict began as a peaceful movement demanding democratic change, but later morphed into a brutal civil war after Assad unleashed a massive crackdown on dissent.
Hezbollah says it is fighting to stop "extremism" from spreading into Lebanon.
Its involvement in the Syrian war has deepened rifts in Lebanon, where most Sunnis support the anti-Assad revolt, while most Shias back the Assad regime.
Hezbollah and the Lebanese army locked down the southern Beirut suburbs because of fears of new violence during Ashoura, allowing only pedestrians and buses to enter.
Most worshippers wore black clothes, while some wore green scarves.
Thousands marched, while others sat in silence in the rain, weeping in memory of the slain imam.
Source: Al Jazeera and agencies