Belgium authorities said an Iranian diplomat was arrested, as well two other people, on suspicion of plotting a bomb attack on a meeting of exiled Iranian opposition groups in France.
A joint statement by Belgium's top prosecutor and the intelligence services said a 38-year-old man and a 33-year-old woman had been intercepted by Belgian agents on June 30.
The two would be charged with attempted terrorist murder and preparation of a terrorist act.
The diplomat at the Iranian Embassy in Vienna was arrested in Germany, according to the Belgian statement, which was released on July 2.
The arrests coincided with a meeting of the National Council of Resistance of Iran, a collection of exiled opposition groups seeking an end to the Shi'ite government in Tehran.
News reports said three other arrests were made in France.
The arrests came as Iranian President Hassan Rohani began a trip to Europe.
In a post to Twitter, Iranian Foreign Minister Mohammad Javad Zarif suggested the arrests were a "false flag" operation.
"How convenient: Just as we embark on a presidential visit to Europe, an alleged Iranian operation and its 'plotters' arrested," Zarif tweeted.