The Hezbollah leader, Sheikh Hassan Nasrallah, has hailed his group's "victory" over Israel, boasting that the group still has 20,000 rockets.
Sheikh Nasrallah had not appeared in public since the conflict
In his first public appearance since the recent conflict, he said Hezbollah would never be disarmed by force and called for a new Lebanese government.
Hundreds of thousands crowded into southern Beirut, heavily bombed during the conflict, to hear the speech.
Israel said the speech showed a lack of respect to the international community.
Waving flags in the yellow and green of Hezbollah, crowds travelled from all over Lebanon to a square in the city's southern suburbs.
Security was tight in the streets around the square.
Thanking the crowd for making the journey to the rally, he praised their courage and said Hezbollah was now stronger than it was before fighting began on 12 July.
The 34-day conflict with Israel ended in a military and a strategic victory for Hezbollah, he told supporters.
"There is no army in the world that can force us to drop our weapons from our hands, from our grip."
Under the terms of the UN-brokered cease-fire that ended the fighting on 14 August, Hezbollah is expected to disarm.
Fighting began on 12 July
Ended 14 August
Israeli dead: 116 soldiers, 43 civilians
Lebanese civilian dead: >1,000
Hezbollah dead: unknown
But Sheikh Nasrallah said the group would only disarm when the Lebanese government was capable of protecting the country, and repeated a Hezbollah call for a new government to replace the administration of Prime Minister Fouad Siniora.
"The building of a just, strong and able state starts first with a serious national unity government," Sheikh Nasrallah said.
The strength of Hezbollah had dealt a severe blow to US plans for a new Middle East peace process, he told the crowds.
The BBC's Arab affairs analyst Magdi Abdelhadi, says the fiery speech consolidated his position as an unrivalled pan-Arab leader and a consummate Lebanese politician.
The speech will go down well in an Arab world where many feel their leaders are either corrupt or weak, he adds
Fighting between Israel and Hezbollah ended on 14 August with a ceasefire that has largely held.
But Israeli government spokesman Mark Regev said Sheikh Nasrallah's speech was a challenge to the international community.
Most of the crowds waved yellow Hezbollah flags
"The international community can't afford to have this Iranian-funded extremist spit in the face of the organised community of nations," he said.
Israel lost 116 soldiers in the fighting, while 43 of its civilians were killed in Hezbollah rocket attacks.
More than 1,000 Lebanese civilians and an unknown number of Hezbollah fighters were killed in the conflict.
Israel failed to achieve its stated war aims of driving Hezbollah fighters from the border, stopping rocket attacks and freeing two of its soldiers captured by Hezbollah in a cross-border raid.
The Beirut rally had been expected to coincide with the final withdrawal of Israeli troops from southern Lebanon, but the Israeli military said on Friday that some troops would remain in Lebanon over the Jewish New Year holiday this weekend.