People from more than 110 cities around the world gathered on Saturday for rallies demanding the release of political prisoners jailed in Iran following its disputed presidential election last month.
International human rights groups, including Amnesty International and Human Rights Watch, backed the "Global Day of Action", in support of the Iranian people.
At least 20 people have been killed in protests following the election, although some human rights groups believe that the toll is much higher.
In Washington, some 3,000 people gathered in front of the Capitol on the National Mall, where Nobel Peace Prize laureate Jody Williams and several human rights activists addressed the crowd.
People wore green and held signs calling for freedom of speech, the release of prisoners of conscience, an end to the violence, and solidarity with the Iranian people.
The organizers were insistent on their call that the rallies concerned human rights in Iran, rather support of any political cause or personality.
A heavy thunderstorm forced the organizers to cut the program short, although a large number of participants braved the rain to hear Iranian singer Dariush sing a patriotic song, based on a poem by Simin Behbani.
The demonstration was live-streamed on YouTube and other Internet sites, and a big screen showed rallies from other cities, including Istanbul and Amsterdam.
In New York, more than 700 demonstrators gathered in Times Square on Saturday, dressed in green, waving green flags, and holding placards and banners.
Nayereh Tohidi, a professor at California State University, Northridge, and Hamid Dabashi, a professor at Columbia University, were among the speakers who called on the international community to support the cause for democracy in Iran.
Following the speeches in Times Square, the demonstrators marched down New York's famed Broadway, towards Iran's permanent mission to the United Nations, and ended with a protest outside of the UN building.
In Los Angeles, home to the largest Iranian community outside Iran, rallies were held at the University of California, Los Angeles [UCLA] and in front of the Federal Building.
Thousands took part in the rally in UCLA, waving green flags and wearing green, and holding photos of those who were killed in the aftermath of the election, reports the Los Angeles Times.
A few blocks away, in front of the Federal Building, more than 100 protesters waving the pre-Islamic revolution Iranian flag staged a rally, calling for the removal of the entire regime.