- Video report by ITV News International Affairs Editor Rageh Omar
Ayatollah Ali Khamenei's sermon comes amid escalating diplomatic tensions between the two countries sparked by the assassination of Iran's second most powerful man Qassem Soleimani in a US airstrike in Baghdad earlier this month.
Addressing a crowd of chanting supporters at the grand mosque in Tehran, the Supreme Leader also used his rare appearance at the weekly prayers to lash out at European countries, including the UK, accusing them of being in America's pocket.
His attack comes after Britain, France and Germany this week triggered a dispute mechanism to try and bring Iran back into compliance with the unraveling 2015 nuclear agreement.
In response, Khamenei said: “These contemptible governments are waiting to bring the Iranian nation to its knees.
“America, who is your elder, your leader and your master, was not able to bring the Iranian nation to its knees. You are too small to bring the Iranian nation to its knees.”
On the same day as Iran's leader led Friday prayers, Canadian President Justin Trudeau has said he will give $25,000 (£14,677) to families of each of the Canadian residents who died in Iran last week.
In response to Soleimani's death, Iran launched a barrage of ballistic missiles targeting US troops in Iraq, without causing serious injuries, an attack Khamenei described as a “blow to America’s image” as a superpower.
The theocratic regime in Iran has come under increasing criticism from Iranians following its response to the Revolutionary Guard accidentally shooting down an Ukrainian airline shortly after it took off from Tehran’s airport, killing all 176 passengers, mostly Iranians and Canadian-Iranians.
Authorities concealed their role in the tragedy for three days, initially blaming the crash on a technical problem.
Their admission of responsibility triggered days of street protests, which security forces dispersed with live ammunition and tear gas.
Mr 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.
Khamenei said the US president was "lying" about his support for the people.
"If you do stand with the Iranian people it is because you want to stick your poisoned dagger into the back of the Iranian nation. Of course you haven't been able to do that so far, and you won't be able to do a damn thing,” he said.
But despite the protests - which come just two months after the regime's brutal crackdown of demonstrations against a hike in fuel prices that left at least 304 people dead - Khamenei said the huge turnout for Solemani's funeral showed Iranian people still supported the Islamic Republic.
Khamenei called the downing of the plane as a "bitter accident" that saddened Iran as much as it made its enemies happy.
Thousands of people attended the Friday prayers, occasionally interrupting his speech by chanting “God is greatest!” and “Death to America!”
Ayatollah Ali Khamenei has held the country’s top office since 1989 and has the final say on all major decisions.
Tensions between Iran and the United States have steadily escalated since President Trump withdrew the US from Iran’s 2015 nuclear deal with world powers, which had imposed restrictions on its nuclear programme in exchange for the lifting of international sanctions.
After Soleimani was killed in a US airstrike in Baghdad, Iran announced it would no longer be bound by the limitations of the nuclear agreement.
The 80-year-old leader openly wept at the funeral of Soleimani and vowed “harsh retaliation” against the United States.