Mansur Osanlu's family and colleagues told Radio Farda that they believe he is in Iranian custody.
They say Osanlu was beaten in the capital on July 10 by a group of unidentified men and taken away in a car.
The International Transport Federation called for Osanlu's unconditional and immediate release.
Osanlu has been detained and jailed several times in the past for organizing protests against the working conditions of bus drivers.
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