Addressing thousands marking Shi'ite Islam's annual Ashura religious commemoration in a heavily secured square in Hezbollah's south Beirut stronghold, Nasrallah said the war in Syria was being fought in defense of the whole region.
In a speech on Tuesday, Nasrallah asserted the Middle East was in a phase of escalating tension and there appeared to be no prospect of a political solution to the war in Syria.
"The regional scene is currently one of tension and escalation, and it does not appear that there are paths for negotiations or solutions," he said in a rare live televised speech before thousands of supporters in Beirut, adding that "the theater (in Syria) was open to more tension, escalation and confrontation."
On Wednesday, black-clad supporters paraded through the streets to mark the 7th-century death of the Prophet Mohammed's grandson Hussein, seen by Shi'ites as a divinely guided leader, and they roared approval of Nasrallah's remarks.
"We answer your call, o Nasrallah!" they chanted.
Since Hezbollah's entry into Syria's civil war on the side of President Bashar Assad, some 1,500 of its fighters have died, say security sources in Lebanon. These have included about 350 this year; their images, often in heroic pose, are displayed on posters in Shi'ite villages across Lebanon.
Hezbollah's part in the war is presented by the movement as both a defense against Sunni Muslim jihadists in Syria who have vowed to exterminate all Shi'ites, and as a way of protecting the regional Iran-backed bloc that has long sustained Hezbollah.
Hezbollah has become an indispensable part of the coalition of forces backed by Iran and Russia that has been critical to keeping Assad in power and battling the myriad groups of Sunni rebels and jihadists striving to depose him.
Some of the rebel groups are backed by Saudi Arabia, whose own military campaign in Yemen was fiercely condemned by Nasrallah and his supporters on Wednesday, as he stepped up rhetoric against the leading Sunni Arab power.
Many of them carried placards denouncing the deaths of Yemeni children in air strikes by the Saudi-led coalition and among the marchers was a group of Yemenis holding up posters of the leader of the Iranian-allied Houthi rebels.
Three young men wearing T-shirts showing the image of their brother who had died in Syria stood by the road as organized groups of Hezbollah supporters in the guise of religious penitents marched past in black clothes and bare feet.
Alaa Nayef Amhaz from Baalbek was 22 when he was killed last year fighting for Hezbollah in the Syrian town of Zabadani, where the army and its Shi'ite allies besieged rebels after months of intense battles.
"The importance of being in Syria is to defend Islam and religion and the nation of the Prophet Mohammed," Amhaz's younger brother Emad, 20, a green scarf over his head and a tattoo circling his wrist, said between puffs on a cigarette.
The most emotive date in the Shi'ite calendar, the death of Hussein at Kerbala is seen as providing an exemplar for how the sect should always stand up against tyranny and social injustice, offering up their lives if necessary.
It has been used as a political call to arms for Shi'ites since Iran's 1979 Islamic Revolution, and was linked at Wednesday's rally to the struggles faced by Hezbollah allies across the region against a Sunni bloc led by Saudi Arabia.
As they marched along Hadi Nasrallah Street, named for their leader's son who was killed in clashes with Israeli troops in 1997, large groups of men clapped their chests with open palms as they chanted support for Hezbollah and its regional allies.
Many of them wore photographs of Iranian Supreme Leader Ayatollah Ali Khamenei - also recognized by Hezbollah as Shi'ite Islam's ultimate authority - tucked into headbands that carried religious slogans.
Behind the marchers, standing on pavements or watching from balconies, were many families, some carrying children on their shoulders, as they enjoyed the day's spectacle.