The suspect, a Vienna-based accredited Iranian diplomat whom opposition groups have named as Assadollah Assadi, 46, was one of several suspects detained in Germany, Belgium and France.
They had allegedly planned to bomb the rally by an exiled Iranian opposition group in Villepinte near Paris that was attended by several US politicians.
German prosecutors said Wednesday that a court had two days earlier remanded Assadi in custody on charges of foreign intelligence activities and conspiracy to murder.
They added that these charges did “not preclude” Assadi's extradition requested by Belgium.
Assadi was believed to be a member of Iran's Ministry of Intelligence and Security, which is tasked with the “observation and fight against opposition groups inside and outside Iran”, the prosecutors' statement said.
They said Assadi had in March ordered a married couple living in Antwerp “to carry out an explosives attack” on the annual rally and had handed them the explosives in a June meeting in Luxembourg.
The couple were arrested in Brussels on the day of the rally by Belgian security services who said they found them in possession of 500 grams (about one pound) of the volatile explosive TATP and a detonator.
They were identified only as Amir S., 38, and Nasimeh N., 33, and as being Belgian nationals of Iranian origin.
French police arrested another three people but later released two of them, French legal sources said.
Teheran has dismissed the alleged bomb plot as a “sinister false flag ploy” designed to discredit Iran at a time when it faces major diplomatic tensions with the United States.
US President Donald Trump over two months ago abandoned the 2015 nuclear deal in which Iran had pledged to halt most nuclear activities in return for sanctions relief.
Washington considers Iran to be the world's biggest state sponsor of terrorism because of its links with the Hezbollah militia in Lebanon, Hamas in the Palestinian territories and other networks in Iraq and Yemen.
News of the suspected bomb plot broke in the week Iranian President Hassan Rouhani was visiting Switzerland and Austria to rally European support for the endangered nuclear agreement.
The Paris rally by the National Council of Resistance of Iran was attended by Trump's personal lawyer, former New York mayor Rudy Giuliani, and former House speaker Newt Gingrich, who both urged “regime change” in Iran.