More than 4,000 people gathered in the Swedish capital Stockholm, police said, waving flags resembling those used in Iran before the shah was ousted in the 1979 Islamic revolution.
“We are demanding respect for human rights in Iran and the release of political prisoners,” said one of the protest organisers, Mehrdad Darvishpour.
After protesting in the city centre, the crowd moved to the Iranian embassy.
Another smaller protest with dozens of people was held in Malmö in the south of the country.
In neighbouring Denmark, nearly a thousand protesters gathered before the country’s parliament, organisers said.
Demonstrations were organised in about 100 cities worldwide on Saturday to denounce human rights violations in Iran.
Ahmadinejad’s re-election has been hotly disputed by his main challenger, Mir Hossein Mousavi, who says his victory was a result of massive vote rigging.
Hundreds of thousands of Iranians poured into Tehran streets to protest their “stolen votes” after the election and security forces launched a crackdown in which at least 20 people died and more than 1,000 were arrested.