Ayatollah Ali Khamenei, Iran’s supreme leader, conducted Friday prayers in Tehran for the first time since 2012 and used the platform to praise the country's retaliatory strike against the U.S. over the killing of one of its top generals. He also appeared to call President Trump a clown who cannot be trusted.

In the rare appearance, Khamenei dismissed the “American clowns” who he said pretend to support the Iranian nation but want to stick their “poisoned dagger” into its back.

“The villainous U.S. government repeatedly says that they are standing by the Iranian people. They lie,” he said. “If you are standing with the Iranian people, it is only to stab them in the heart with their venomous daggers.”

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He also insisted Iran would not bow to U.S. pressure after months of crushing sanctions and called Washington’s decision to kill Gen. Qassem Soleimani a “cowardly” hit.

“The shameless U.S. government was disgraced by calling Martyr Soleimani a terrorist when he was recognized by everyone as the most prominent and powerful commander in the fight against terrorism,” he said. “The U.S. government killed Martyr Soleimani, not in the battlefield but thievishly and cowardly.”

Iran, in response to the Jan. 8 airstrikes that billed Soleimani in Baghdad, launched a barrage of ballistic missiles targeting U.S. troops in Iraq. While there were no casualties, the U.S. military said 11 service members were flown out of Al Assad Air Base in Iraq and treated for concussion symptoms.

“The fact that Iran has the power to give such a slap to a world power shows the hand of God,” Khamenei said.

“More important and greater than a military strike, it was a blow to the dignity and awe of the U.S. as a superpower.”

As Iran’s Revolutionary Guard braced for a possible American counterattack, it mistakenly shot down a Ukrainian passenger jetliner shortly after it took off from Tehran’s international airport. All 176 people on board – mostly Iranians – were killed.

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Khamenei called the shootdown of the plane a “bitter accident” that saddened Iran as much as it made its enemies happy.

Authorities concealed their role in the tragedy for three days, initially blaming the crash on a technical problem. When it came, their admission of responsibility triggered days of street protests, which security forces dispersed with live ammunition and tear gas.

Khamenei has held the country's top office since 1989 and has the final say on all major decisions. The 80-year-old leader openly wept at the funeral of Soleimani and vowed “harsh retaliation” against the United States.

Thousands of people attended the Friday prayers, occasionally interrupting his speech by chanting “God is greatest!” and “Death to America!”

Khamenei told the crowd Friday that President Trump is not to be trusted and only pretends to support the Iranian people. He said Western countries are too weak to "bring Iranians to their knees." He said Iran was willing to negotiate, but not with the U.S.

Tensions between Iran and the United States have steadily escalated since Trump withdrew from the nuclear accord, which had imposed restrictions on Iran's nuclear program in exchange for the lifting of international sanctions.

The U.S. has since imposed crippling sanctions on Iran, including its vital oil and gas industry, pushing the country into an economic crisis that has ignited several waves of sporadic, leaderless protests. Trump has openly encouraged the protesters — even tweeting in Farsi — hoping that the protests and the sanctions will bring about fundamental change in a longtime adversary.

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Khamenei last delivered a Friday sermon in February 2012, when he called Israel a “cancerous tumor” and vowed to support anyone confronting it. He also warned against any U.S. strikes on Iran over its nuclear program, saying the U.S. would be damaged “10 times over.”

His decision to lead the prayers was seen as a "symbolically significant act," one usually reserved for an important message to the people, a Middle East scholar told The Washington Post.

Fox News' Adam Shaw, Louis Casiano, Greg Norman, and the Associated Press contributed to this report.