He said his appearance in the suburb of Dahiya was a message to those who believed they could "threaten us".
He has rarely been seen in public since Hezbollah's war with Israel in 2006.
The 34-day conflict left more than 1,200 Lebanese dead, most of them civilians. Some 160 Israelis, most of whom were soldiers, were also killed.
'Regional change' On Tuesday, the Hezbollah leader was filmed walking through a crowd in Dahiya - one of the group's strongholds - surrounded by bodyguards.
"I wanted to be with you for few minutes... to renew our pledge and for the world to hear us," he told the tens of thousands of people there.
The crowd chanted, "Death to Israel" in response.
After only a few minutes, Sheikh Nasrallah told his supporters that he would soon reappear on a giant screen and left the podium.
In the speech, he promised that his group's military wing, the Islamic Resistance, would continue to be armed regardless of "regional change".
"A message to all those who are conspiring against the resistance and banking on change... We will never let go of our arms," he said. "Day after day, the resistance gains more fighters, trains better fighters and arms even more heavily."
He also offered his support to the Syrian government and accused the United States of plotting to destroy Syria.
"We support the reforms in Syria and we stand with the regime against the resistance movement," he said.
"There are some people who do not want reforms, security and stability in Syria, and neither civil peace nor dialogue. There are people who want to destroy Syria to make up for their defeat in Iraq. Syria is a partner in defeating the Americans in Iraq."
The day of Ashura is marked by Muslims as a whole, but for Shia Muslims it is a major religious festival which commemorates the martyrdom at Karbala of Imam Hussein, a grandson of the Prophet Muhammad.