Trump posted on Twitter two clips of his speech to the UN General Assembly in September in which he took aim at the Iranian regime, which Washington has held out as its top adversary in the Middle East.

"Oppressive regimes cannot endure forever, and the day will come when the Iranian people will face a choice," he tweeted, quoting from the speech.

"The world is watching!"

Trump's posts, his second time addressing the subject in as many days, came as several hundred anti-government demonstrators clashed with police at the University of Tehran in a third straight day of protests.

Oppressive regimes cannot endure forever, and the day will come when the Iranian people will face a choice. The world is watching! pic.twitter.com/kvv1uAqcZ9 — Donald J. Trump (@realDonaldTrump) December 30, 2017

Hundreds of counter-protesters also massed outside the entrance to the university, chanting "Death to the seditionists" in a show of support for the regime.

Videos shared by social media users outside Iran but which could not be independently verified claimed to show thousands marching peacefully against the regime in several cities including Khorramabad, Zanjan and Ahvaz, with chants of "Death to the dictator."

"The entire world understands that the good people of Iran want change, and, other than the vast military power of the United States, that Iran's people are what their leaders fear the most," Trump said, again quoting from the UN speech.

Trump also tweeted in support of the protesters late Friday, prompting Iran's foreign ministry spokesman Bahran Ghasemi to dismiss his remarks as "opportunistic."