Ahmad Khatami also told worshippers attending Eid prayers in Tehran that President Donald Trump's offer for direct talks with Iranian leaders was unacceptable.
"Americans say you should accept what we say in the talks. So this is not negotiation, but dictatorship. The Islamic Republic and the Iranian nation would stand up against dictatorship," Khatami was quoted as saying by Mizan news agency.
Khatami — who is also a member of Iran's Council of Experts, which is tasked with designating and dismissing the country's supreme leader — is known for his hardline positions on a number of issues. He previously supported the call for a death sentence for author Salman Rushdie, denounced student protesters who partook in Iran's 2009 election protests as "enemies of God," and in 2006 asked Pope Benedict XVI to "fall on his knees in front of a senior Muslim cleric and try to understand Islam." For a senior Iranian cleric, his comments are not particularly surprising.
After imposing wide-ranging sanctions on Tehran following withdrawal from the 2015 Iran nuclear deal, which lifted economic restrictions on the country in exchange for curbs on its nuclear program, Trump has offered talks with Iran's leadership without preconditions. Tehran has so far rejected the idea.
Iran has long been a target of Trump's animus, particularly over its involvement in regional affairs across Lebanon, Syria, Yemen and Iraq as well as its continued ballistic missile testing, in violation of UN resolutions. The nuclear deal, however, signed along with Russia, China and EU member states, appeared to be working, the International Atomic Energy Association repeatedly verified Iran's compliance.
Experts see heightened attacks on U.S. interests in the region and intensified interference in broader Middle Eastern conflicts as a likely avenue of retaliation for Tehran. Its Quds Force, the external wing of Iran's Revolutionary Guard Corps (IRGC), is an active presence in Syria and provides support to proxies in Lebanon, Iraq, and to a growing or lesser extent, Yemen and Afghanistan.
Iran's engagement in Syria is of particular concern to U.S. ally Israel, which sees the country's attempt at gaining a foothold on its border as an existential threat. Israeli officials believe any territory held by Iranian forces close to the border will serve as bases from which Shia proxies like Hezbollah can launch attacks against it.
As a result, Israeli forces have already carried out several airstrikes over parts of Syria, killing a number of Iranian and Syrian military and paramilitary personnel. Some of the strikes were in response to Iranian armed drones penetrating Israeli airspace. Despite threats, Iran has so far not struck back directly at Israel — likely due to its disinclination to get into an all-out war with the region's most elite military.
Russia, the primary backer of Syrian President Bashar Assad along with Iran, has offered Israel a "buffer zone" of 100 kilometers from its border in which Iranian forces could not operate. Jerusalem has so far rejected the idea as not nearly substantial enough for its security considerations.
Iran is in the midst of a currency crisis brought on by Washington's sanctions and years of economic mismanagement and corruption on the part of the regime. As inflation soars and Iranians hoard goods and gold, protests cropping up around the country testify to the frustration and anger of ordinary Iranians, many of whom take issue with their country's excessive spending on foreign campaigns. The UN estimates that Tehran has spent $6 billion per year to prop up the Syrian regime alone.
Still, the protests aren't likely to pose a serious threat to the regime, owing to their sporadic and disorganized nature and to the powerful repressive capacity of the Iranian government. Some Trump administration officials, particularly National Security Advisor John Bolton and Secretary of State Mike Pompeo, have voiced their support for the protests and their potential to spur regime change.
While this may be a goal of some in the White House, whether that takes a military form is so far unlikely, experts say. Bolton repeatedly called for bombing Iran prior to his appointment; officials at the Pentagon, meanwhile, have been more guarded.
Secretary of Defense James Mattis is said to be one of the few top administration members holding back an attack on Iran. While very critical of what he terms the country's "malign influence" and threats to U.S. interests, he is also aware of the stratospheric cost a military conflict with Iran would incur, favoring economic and diplomatic pressure.
Still, he has made clear that no option is off the table.
"We maintain military options because of Iran's bellicose statements and threats," Mattis said during a Senate testimony in May. "And those plans remain operant."
—Reuters contributed to this article.