The U.S. has reimposed a naval blockade on Iran and intensified its airstrike campaign in retaliation for Tehran's attacks on ships trying to pass through the Strait of Hormuz.
Crowds of hundreds of thousands chanted "Death to America" and "Death to Israel," as they called for revenge over the Feb. 28 attack.
Iran has begun a dayslong funeral for the late Supreme Leader Ayatollah Ali Khamenei, months after an airstrike killed him at the start of the war.
After four days of trading strikes, both sides appeared to pause their attacks Monday.
The statement issued Sunday also threatened a "complete halt" to negotiations to end the war if Washington continues its attacks.
The U.S. military has launched a second round of airstrikes targeting Iran at President Donald Trump's direction.