Iran's authorities indicated on Saturday they could intensify their crackdown on the biggest anti-government demonstrations in years, with the Revolutionary Guards blaming unrest on terrorists and vowing to safeguard the governing system.

US President Donald Trump, who has repeatedly threatened to intervene in recent days, posted on social media on Saturday: "Iran is looking at FREEDOM, perhaps like never before. The USA stands ready to help!!!"

There were fresh reports of violence across Iran, although an internet blackout made it difficult to assess the full extent of unrest.

After nightfall on Saturday, new videos posted online purported to show fresh protests in a number of neighbourhoods in the capital Tehran and several cities, including Rasht in the north, Tabriz in the northwest and Shiraz and Kerman in the south. Reuters could not immediately verify the latest videos.

The exiled son of Iran's last shah, who has emerged as a prominent voice in the fragmented opposition, made his strongest call yet for the protests to broaden into a revolt to topple the clerical rulers.

State media said a municipal building was set on fire in Karaj, west of Tehran, and blamed "rioters". State TV broadcast footage of funerals of members of the security forces it said were killed in protests in the cities of Shiraz, Qom and Hamedan.