Many in the crowd wore green wristbands and headbands -- Mr. Mousavi's campaign color. The marchers carried signs but were mainly silent to emphasize the peaceful theme of the demonstration.
Mr. Mousavi, a reformist, has called for another demonstration Thursday aimed at getting officials to annul the election results, which he described as a "shameful fraud." In a statement on his Web site, he has asked his supporters to gather in mosques or hold peaceful marches to honor those killed in post-election violence.
Iran's Revolutionary Guards cracked down further on the media Wednesday, threatening legal action against Web sites that create "tension." As part of the media crackdown, the press freedom group, Reporters Without Borders, says at least 11 Iranian journalists have been arrested since Friday's election.
Meanwhile, rights groups say numerous opposition members have been arrested in recent days. That includes Ebrahim Yazdi, the General Secretary of the banned Freedom Movement Party, Nehzat e Azadi, who was allegedly arrested at a hospital in Tehran.
Reporters and photographers of foreign news organizations have already been barred from covering "unauthorized" political events and demonstrations.
But a high-profile display of opposition support was televised in Iran when at least four members of Iran's national football team wore green wristbands during the first half of a World Cup qualifying match in Seoul, South Korea.
The election, which handed a landslide victory to President Mahmoud Ahmadinejad, has triggered Iran's most serious unrest since the 1979 Islamic revolution. Iran's Press TV says post-election violence has left at least eight people dead.
Groups supporting Mr. Ahmadinejad, a conservative, have also staged large demonstrations.
Government officials say Mr. Ahmadinejad won re-election with 63 percent of the ballots last Friday, compared with 34 percent for Mr. Mousavi.
Some information for this report was provided by AFP, AP and Reuters.