A number of labor activists have been detained in the Iranian capital as they gathered to observe International Workers' Day on May 1.
The activists were among dozens of workers who were aiming to stage a protest outside the Labor Ministry and also at another location, in western Tehran.
Labor activist Davood Razavi told RFE/RL's Radio Farda that some 23 protesters were detained.
He said the crackdown demonstrates the lack of tolerance of the Iranian government for even a small form of dissent.
"The drivers wanted to distribute flowers and cookies," said Razavi in a telephone interview from Tehran.
Ebrahim Madadi, a senior member of the Union of the Workers of Tehran and Suburbs Bus Company, is among those detained.
He was reportedly beaten up and transferred to an unknown location.
Contributions from Roozbeh Bolhari in Prague and Golnaz Esfandiari in Washington