Ayatollah Ali Khamenei has called Israel a "rabid dog and rapacious wolf".
In his Eid al-Fitr address, the Iranian Supreme Leader said: "This rabid dog, this rapacious wolf, has attacked innocent people and humanity must show a reaction. This is genocide, a catastrophe of historical scale."
Khamenei also condemned the Western world for failing to act in order to save the lives of hundreds of Palestinian civilians in the ongoing Israel-Gaza conflict.
Khamenei, who wields ultimate authority in Iranian political affairs, exhorted the Muslim world to unite in supporting the Palestinian cause and arm them.
"Everyone, whoever has the means, especially in the Islamic world, they should do what they can to arm the Palestinian nation ... the Zionist regime deeply regrets starting this [war] but has no way out.
"They [Israeli forces] have been pounding innocent people day and night and these men, women and children are defending themselves with minimum means, and now Americans and Europeans want to take even that away ... so that those merciless beasts could pound without qualm."
Reaffirming Iran's support for Hamas, Iranian President Hasssan Rohani in his message criticised Israel for its continuing offensive in Gaza.
Sayed Hassan Nasrallah, the leader of the Lebanon-based, Iran-backed militant group Hezbollah, hailed Iran for defending Palestinian rights.