Reza Pahlavi, the exiled son of Iran's last shah, addressed a crowd of 200,000 people in Munich on Saturday, telling them he was ready to lead the country to a "secular democratic future".
Pahlavi urged Iranians at home and abroad to continue demonstrating, calling on them to chant slogans from their homes and rooftops at 8pm (1630 GMT) on Saturday and Sunday, to coincide with the protests in Germany and elsewhere.
Thousands more people took part in solidarity demonstrations in cities including Los Angeles, Washington, Toronto, Tel Aviv, Lisbon, Sydney and London.
The rallies came as Washington prepared for a fresh round of talks with Iranian government representatives in Geneva this week despite Donald Trump's insistence that a change of power in Tehran would be the "best thing".
Pahlavi - who is based in the US and who has not returned to Iran since before the 1979 Islamic revolution that ousted the monarchy - told the crowd in Munich that he could lead a democratic handover.
"I am here to guarantee a transition to a secular democratic future," he said. "I am committed to be the leader of transition for you so we can one day have the final opportunity to decide the fate of our country through a democratic, transparent process to the ballot box."
"The Iranian regime is a dead regime," a 62-year-old protester originally from Iran who gave his name only as Said told Agence France-Presse (AFP). "It must be game over."
Speaking on Friday as he sent a second aircraft carrier to the Middle East to ratchet up military pressure on Tehran, the US president said a change of government in Iran would be the "best thing that could happen".
Trump had earlier threatened military intervention to support a wave of protests in Iran that peaked in January and were met by a violent crackdown that rights groups say killed thousands.
Although the president had initially said the US was "locked and loaded" to help demonstrators when the government crackdown began, he has more recently focused his military threats on Tehran's nuclear programme, which US forces struck last June during Israel's unprecedented 12-day war with Iran.
Representatives of Iran and the US, which have had no diplomatic relations since shortly after the 1979 revolution, held talks on the nuclear programme last week in Oman.
On Sunday, a Swiss foreign ministry spokesperson told AFP that Oman would host talks in Geneva next week, without providing further details.
Videos verified by AFP showed people in Iran this week chanting anti-government slogans despite the crackdown, as the clerical leadership celebrated the anniversary of the Islamic revolution.
According to the US-based Human Rights Activists' News Agency, at least 7,010 people, mostly protesters, were killed in the crackdown, though they and other rights groups warn the toll is likely far higher. It said more than 53,845 people had been arrested.
The Iranian opposition remains divided and Pahlavi has faced criticism for his support for Israel, making a highly publicised visit in 2023 that fractured an attempt to unify opposition camps. He has also never distanced himself from his father's autocratic rule.