The rally attracted about 700 people - a far cry from the hundreds of thousands of protesters who have lined the streets of Tehran in recent days. Most were clad in green, the color Mousavi used during his campaign for president.
Rally-goers held signs, lit candles and peacefully chanted - at times in Farsi - as twilight turned to nighttime.
Event organizer Amid Amidi, a first generation Iranian-American, said he conceived the idea for the rally to show support for the people risking their lives while protesting on the streets of Tehran.
"The dam is leaking in Iran," Amidi said. "They're on the verge of . . . a democratic and free government."
Amidi said he organized the demonstration in about 24 hours using Twitter and Facebook.