Saturday's global day of action across about 100 cities in six continents was organized by United For Iran and supported by several human rights groups, including Human Rights Watch, Amnesty International and Reporters Without Borders.
In London, England, protesters waved green flags and wore green wristbands -- the color is symbolic of the opposition movement in Iran.
The British protesters outside the Iranian Embassy in London wanted to show solidarity for those Iranians "who feel too intimidated, too fearful" to go back out on the streets to protest, reported CNN Correspondent Paula Newton.
Several of the demonstrations, especially in the United States and Europe, called for Western governments to be more vocal about the reported human rights violations in Iran.
"Enough is enough," said Parviz Chahi, a demonstrator in London. "How many people do they have to sacrifice?"
The demonstrations on Saturday condemned the Iranian government's violent response to citizens who claim the June 12 re-election of President Mahmoud Ahmadinejad was fraudulent. Hundreds of thousands of Iranians took to the streets after the election, and many were met with brutality from security forces that resulted in death or serious injuries.
Some protesters in Tehran and other Iranian cities were arrested and put in jail, where they remain despite pleas by families and friends.
"These acts are a direct assault on the Iranian people's human and civil rights -- and those of everyone who supports these ideals around the world," United For Iran said on its Web site.
"Our only aim is to condemn the widespread and systematic violations of the Iranian people's human rights and to call for full restoration of their human and civil rights," the group says.
Saturday's events came ahead of Ahmadinejad's swearing-in ceremony, scheduled for the first week of August. Send your iReports from Saturday's rallies
In Berlin, Germany, about 2,000 people turned out to rally for Iranians. A moment of silence was held, and about 40 people have been participating in a hunger strike over the past three days, CNN Correspondent Frederik Pleitgen reported.
Germany is one of Iran's major trade partners, prompting the Berlin protesters to call on their government to get tough with Tehran over the Islamic republic's human rights record and nuclear ambitions.
United For Iran said it wants U.N. Secretary-General Ban Ki-moon to send a delegation to Iran to investigate the fate of political prisoners; the immediate and unconditional release of all political prisoners and prisoners of conscience, including journalists; an end to state-sponsored violence; freedom of the press and Iran's adherence to international agreements it has signed.