US President Donald Trump, once again backing the ongoing protests in Iran has warned the authorities in Tehran over their brutal crackdown on protesters, and hailed the Iranian people as "strong" and "brave."

His comments came as millions took to the streets in various parts of the country, protesting against the theocratic regime and shouting for the ouster of the Supreme Leader Ali Khamenei led regime, in favour of exiled Prince Reza Pahlavi.

In response, Iranian security personnel upped the ante of their crowd control measures with ruthless suppression, detaining thousands and killing several protesters amid the nationwide demonstrations on Thursday.

Trump, talking to podcaster Hugh Hewitt, said that the Iranian leadership "have been told very strongly… that if they do that, they're going to have to pay hell."

This is the third time since the start of protests on December 28 that Trump has warned Tehran against killing demonstrators, threatening possible US intervention.

Vice President JD Vance, backing Trump's statement, said that the administration stands by "anybody who is engaged in peaceful protests" and seeking to exercise "their rights of free association and to have their voices heard," including in Iran.

"Obviously, the Iranian regime has a lot of problems, as the President of the United States has said, the smartest thing for them to have done, it was true two months ago, it's true today, is for them to actually have a real negotiation with the United States about what we need to see when it comes to their nuclear program," Vance said to reporters at the White House.

"I'll let the President speak to what we're going to do in the future. But we certainly stand with anybody across the world, including the Iranian people, who are advocating for their rights," he added.

Addressing Iranians directly, he urged them to "feel strongly about freedom," and said "There's nothing like freedom. You're brave people. It's a shame what's happened to your country."

Protesters in Iran have appealed directly to Trump for protection, with human rights groups saying that at least 36 people have been killed since the protests began on December 28, while more than 2,000 people have been arrested or detained.

A nationwide internet blackout was imposed on the country on Thursday, after protests intensified following a video released by Prince Pahlavi, who urged protesters to carry on, leading to a surge in demonstrations, leading to the regime imposing an internet shutdown.

Asked if he would meet exiled Prince Pahlavi, Trump said he is still monitoring the situation in the country, before he meets the exiled prince or endorses any potential future figure for Iran.

"Well, I've watched him, and he seems like a nice person, but I'm not sure that it would be appropriate at this point to do that as President," Trump responded.

"I think that we should let everybody go out there and see who emerges. I'm not sure necessarily that it would be an appropriate thing to do."