US President Donald Trump warned of unspecified "very strong action" if Iranian authorities go ahead with threatened hangings of some protesters, with Tehran calling American warnings a "pretext for military intervention".
International outrage has built over the crackdown that a rights group said has likely killed thousands during protests posing one of the biggest challenges yet to Iran's clerical leadership.
Iran's UN mission posted a statement on X, vowing that Washington's "playbook" would "fail again".
"US fantasies and policy toward Iran are rooted in regime change, with sanctions, threats, engineered unrest, and chaos serving as the modus operandi to manufacture a pretext for military intervention," the post said.
Iranian authorities have insisted they had regained control of the country after successive nights of mass protests nationwide since.
Rights groups accuse the government of fatally shooting protesters and masking the scale of the crackdown with an internet blackout that has now surpassed the five-day mark.
New videos on social media, with locations verified by AFP, showed bodies lined up in the Kahrizak morgue just south of the Iranian capital, with the corpses wrapped in black bags and distraught relatives searching for loved ones.
Trump -- who earlier told the protesters in Iran that "help is on its way" -- said Tuesday in a CBS News interview that the United States would act if Iran began hanging protesters.
Tehran prosecutors have said Iranian authorities would press capital charges of "moharebeh", or "waging war against God", against some suspects arrested over recent demonstrations.
"We will take very strong action if they do such a thing," said the American leader, who has repeatedly threatened Iran with military intervention.
"When they start killing thousands of people -- and now you're telling me about hanging. We'll see how that's going to work out for them," Trump said.
The US State Department on its Farsi language X account said 26-year-old protestor Erfan Soltani had been sentenced to be executed on Wednesday.
"Erfan is the first protester to be sentenced to death, but he won't be the last," the State Department said, adding more than 10,600 Iranians had been arrested.
Rights group Amnesty International called on Iran to immediately halt all executions, including Soltani's.
Trump urged on his Truth Social platform for Iranians to "KEEP PROTESTING", adding: "I have cancelled all meetings with Iranian Officials until the senseless killing of protesters STOPS. HELP IS ON ITS WAY."
It was not immediately clear what meetings he was referring to or what the nature of the help would be.
- 'Rising casualties' -
European nations also signalled their anger over the crackdown, with France, Germany and the United Kingdom among the countries that summoned their Iranian ambassadors, as did the European Union.
"The rising number of casualties in Iran is horrifying," said EU chief Ursula von der Leyen, vowing further sanctions against those responsible.
Norway-based NGO Iran Human Rights said it had confirmed 734 people killed during the protests, including nine minors, but warned the death toll was likely far higher.
"The real number of those killed is likely in the thousands," IHR's director Mahmood Amiry-Moghaddam said.
Iranian state media has said dozens of members of the security forces have been killed, with their funerals turning into large pro-government rallies.
Authorities in Tehran have announced a mass funeral ceremony in the capital on Wednesday for the "martyrs" of recent days.
Amir, an Iraqi computer scientist, returned to Baghdad on Monday and described dramatic scenes in Tehran.
"On Thursday night, my friends and I saw protesters in Tehran's Sarsabz neighbourhood amid a heavy military presence. The police were firing rubber bullets," he told AFP in Iraq.
Reza Pahlavi, the US-based son of Iran's ousted shah, called on the military to stop suppressing protests.
"You are the national military of Iran, not the military of the Islamic Republic," he said in a statement.
- 'Serious challenge -
The government on Monday sought to regain control of the streets with mass nationwide rallies that supreme leader Ayatollah Ali Khamenei hailed as proof that the protest movement was defeated, calling them a "warning" to the United States.
In power since 1989 and now aged 86, Khamenei has faced significant challenges, most recently the 12-day war in June against Israel, which forced him to go into hiding.
Analysts have cautioned that it is premature to predict the immediate demise of the theocratic system, pointing to the repressive levers the leadership controls, including the Revolutionary Guard Corps, which is charged with safeguarding the Islamic revolution.
Nicole Grajewski, professor at the Sciences Po Centre for International Studies, told AFP the protests represented a "serious challenge" to the Islamic republic, but it was unclear if they would unseat the leadership, pointing to "the sheer depth and resilience of Iran's repressive apparatus".
