Iranian President Hassan Rouhani said Thursday that the chaos unleashed on the US Capitol by US counterpart Donald Trump's supporters exposed the fragility of Western democracy.

"What we saw in the United States yesterday (Wednesday) evening and today shows above all how fragile and vulnerable Western democracy is," Rouhani said in a speech broadcast by state television.

"We saw that unfortunately the ground is fertile for populism, despite the advances in science and industry.

"A populist has arrived and he has led his country to disaster over these past four years.

"I hope the whole world and the next occupants of the White House will learn from it."

Rouhani said he hoped for a change of direction from the incoming administration of US President-elect Joe Biden.

He urged the new administration "to make up (for the past) and restore the country to a position worthy of the American nation, because the American nation is a great nation".

"May they return to reason, legality and their obligations. It's for their own benefit and the good of the world," he said.

Despite routine references to the United States as the "Great Satan" in official rhetoric, it is not the first time that an Iranian president has called America a "great nation".

Rouhani, a relative moderate in Iranian politics, presided over negotiations for a landmark 2015 nuclear agreement with major powers that Trump abandoned in 2018.

He has staked his reputation on a diplomatic opening to the incoming Biden administration to try to rescue the deal.

The Rouhani government's top diplomat said Trump was a "security concern" for the international community.

"A rogue president who sought vengeance against his OWN people has been doing much worse to our people—and others—in the past 4 years," Foreign Minister Mohammad Javad Zarif tweeted.

"What's disturbing is that the same man has the UNCHECKED authority to start a nuclear war; a security concern for the entire int'l community."