Iranian labour activist Zabiollah Bagheri was arrested in Esfahan this week, according to the Committee of Human Rights Reporters
Bagheri was summoned by factory security as he was leaving work and was taken to an undetermined location by three officials in plain clothes.
The reason for the arrest and his whereabouts remain unknown.
The report indicates that in recent months, Bagheri was involved in organizing labour protests against the non-payment of wages and the suspension of worker benefits.
Bahgheri was previously arrested during the funeral ceremony for dissident cleric Ayatollah Montazeri in December of 2009.
The Kaleme opposition website reports that with the approach of May Day, the Islamic Republic has been ratcheting up pressure on labour activists and groups, and many workers have been summoned and threatened with arrest if they are caught participating in any labour event to mark the occasion.
The government has remained silent about a formal request from labour organizations to organize a march on May Day.