Iranian intelligence ministry agents arrested five members of Tehran's bus drivers' union after they visited the home of their imprisoned chief Mansour Ossalou. ISNA identified the detainees as Ebrahim Madadi, Yagoub Salimi, Davoud Razavi and Ghojari and Homayoun Jaberi.
Ossalou has been sentenced to four years in prison for acting against national security and to an additional year for disseminating propaganda against the Islamic Republic.
Rights group Amnesty International, the Brussels-based International Trade Union Confederation (ITUC) and the International Transport Workers' Federation held protests in European capitals on Thursday calling for his release.
They also called for the release of Mahmoud Salehi, a prominent trade unionist and former head of the bakers' union in Iran's Kordestan province, who was rearrested in April after serving previous jail terms.