After a two-day calm in Tehran, protesters returned to eastern and western neighbourhoods, chanting slogans against the Islamic Republic. In addition, night protests have been reported in the major cities of Mashhad (northeast), Shiraz (south), and Zahedan (southeast).
Trump threatened Tehran with coming to the rescue of Iranians.
"We are locked and loaded and ready to go," he said in a social media post.
The United States bombed Iranian nuclear facilities in June, joining an Israeli air campaign that targeted Tehran's atomic programme and military leadership.
Responding to Trump's comments, top Iranian official Ali Larijani warned that US interference in domestic Iranian issues would amount to a destabilisation of the entire Middle East. Iran backs proxy forces in Lebanon, Iraq and Yemen.
Reza Pahlavi, the son of the shah who was overthrown in the 1979 revolution, thanked the US president for his "strong leadership" in a post on X.
The comments came as a local official in western Iran, where several deaths were reported, was cited by state media as warning that any unrest or illegal gatherings would be met "decisively and without leniency".
In a Friday letter to the UN secretary-general and president of the Security Council seen by Reuters, Iran's UN Ambassador Amir-Saeid Iravani called for the Security Council to condemn Trump's statements.
"Iran will exercise its rights decisively and proportionately. The United States of America bears full responsibility for any consequences arising from these unlawful threats and any ensuing escalation," he said in the letter.
This week's protests over soaring inflation are so far smaller than some previous bouts of unrest in Iran, but have spread across the country, with deadly confrontations between demonstrators and security forces focused in western provinces.
State-affiliated media and rights groups have reported at least 10 deaths since Wednesday, including one man who authorities said was a member of the Basij paramilitary force affiliated with the elite Revolutionary Guards.
The Islamic Republic's clerical leadership has seen off repeated eruptions of unrest in recent decades, often quelling protests with heavy security measures and mass arrests. But economic problems may leave authorities more vulnerable now.
This week's protests are the biggest since nationwide demonstrations triggered by the death of a young woman in custody in 2022 paralysed Iran for weeks, with rights groups reporting hundreds killed.
Video verified by Reuters showed dozens of people gathered in front of a burning police station overnight, as gunshots sporadically rang out and people shouted "shameless, shameless" at the authorities.
In the southern city of Zahedan, where Iran's Baluch minority predominates, the human rights news group Hengaw reported that protesters had chanted slogans including "Death to the dictator".
Hengaw has reported at least 80 arrests so far over the unrest, mostly in the west, and including 14 members of Iran's Kurdish minority.
State television also reported the arrest of an unspecified number of people in another western city, Kermanshah, accused of manufacturing petrol bombs and homemade pistols. Iranian media also said two heavily armed individuals were arrested in central and western Iran before they could carry out attacks.
The deaths acknowledged by official or semi-official Iranian media have been in the small western cities of Lordegan and Kuhdasht. Hengaw also reported that a man was killed in Fars province in central Iran, though state news sites denied this.
Reuters could not verify all the reports of unrest, arrests or deaths.
with EFE and DPA
