The witness said family members, including wives of people arrested after the June vote, held pictures of detainees, including former deputy interior minister Mostafa Tajzadeh and former government spokesman Abdullah Ramezanzadeh.
The gathering was peaceful and there was no sign of police, the witness said. The relatives did not chant slogans, but called for the detainees to be freed in writing on placards, the witness added.
Thousands of people were arrested after the June 12 presidential election, which sparked huge street protests.
The moderate opposition says the poll was rigged to secure hardline President Mahmoud Ahmadinejad's re-election. The authorities deny it.
Most of the detainees have since been released, but more than 100 senior reformers, activists, journalists and others, have been put on trial accused of fomenting post-election unrest. The opposition has denounced the court sessions as "show trials."
The authorities have portrayed the post-election street demonstrations, which were quelled by the elite Revolutionary Guards and an allied Islamic militia, as a foreign-backed bid to undermine the Islamic Republic.
(Writing by Fredrik Dahl; editing by Janet Lawrence)
