As he appeared on stage wearing a black robe and turban, the crowd seen in a live broadcast on Hezbollah's Al-Manar television began cheering wildly, as they apparently had not expected to see him.
The head of the Shiite militant group, whose forces are fighting in Syria alongside the troops of President Bashar al-Assad, usually addresses supporters via video link for fear of assassination by arch-foe Israel.
Nasrallah had not been seen in public since July, when he attended a rally to show support for Israeli-blockaded Gaza Strip.
But Monday's appearance in the southern suburbs of Beirut, a Hezbollah stronghold, was his sixth since his group fought Israel in a devastating and deadly war in 2006.
Nasrallah has topped Israel's most wanted list since even before that 34-day war, and his group has also made enemies with Sunni jihadists from Syria since its involvement in that country's conflict.
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Hezbollah's fighters clashed with jihadists in eastern Lebanon in October, and its strongholds have come under repeated bomb attacks over its involvement in the Syrian conflict.
Nasrallah's address Monday came ahead of the peak of Ashura, a festival that marks the killing of Imam Hussein, one of the most revered figures of Shiite Islam and grandson of the Prophet Mohammed.
Hussein 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 Shiite and Sunni sects.
Security is usually tight for the event.
Lebanese police will close off the Shiite-majority southern suburbs of Beirut for Ashura from midnight until the end of the commemorations.
Hezbollah is planning to hold a massive rally in the southern suburbs of Beirut on Tuesday to mark Ashura, and Nasrallah is due to address the crowds again.