Armored vehicles moved in to arrest the demonstrators, "shoving them in like livestock," according to a source in Teheran.
Mir Hossein Mousavi, the defeated presidential candidate who says the vote was rigged, has not been seen since last Thursday and is widely believed to be in hiding.
But the protests were organized through his official Web site, and he has previously sent messages urging the demonstrators to continue their rallies.
From the rooftops and windows of Teheran apartment buildings, residents are keeping up their protest slogans, with reports of shouts of "Death to the Islamic Republic."
With Mousavi out of view, and all prominent reformist figures under arrest or also in hiding, the opposition had no visible leadership.