Iran "strongly" condemned the sentencing Thursday of one of its diplomats to 20 years in prison by a Belgian court for plotting a thwarted 2018 bombing of an opposition rally outside Paris.

"The Islamic Republic of Iran strongly condemns the announcement by a court in Antwerp, Belgium, according to which Mr. Assadollah Assadi... was sentenced to 20 years in prison," said foreign ministry spokesman Saeed Khatibzadeh. 

Tehran considers the "judicial process and verdict" as "illegal and in flagrant violation of international law", he added.

Iran had warned even before the conviction that it would not recognise the trial or the verdict, denying any official role in the plot and insisting Assadi ought to have enjoyed immunity from prosecution.

But investigators concluded he was an Iranian agent working under diplomatic cover.

The Belgian court found that Assadi, now 49, was attached to the Iranian embassy in Austria when he supplied explosives for an attack that was planned for a June 30, 2018 rally in France of the exiled opposition group the National Council of Resistance in Iran (NCRI).

He was charged with "attempted murders of a terrorist nature" and "taking part in the activity of a terrorist group".

"Unfortunately, Belgium and certain European countries, under the hostile influence of the Monafeqin terrorist group in Europe have taken such an illegal and unjustifiable action," Khatibzadeh said.

The term Monafeqin, hypocrites in Persian, is used by Iran to refer to the armed wing of the NCRI.

Assadi's trial was conducted under tight security in the Belgian port of Antwerp and has further poisoned already tense relations between Tehran and European capitals.

Three of his accomplices, dual Iranian-Belgian nationals, were also arrested and on Thursday were given jail terms of between 15 and 18 years and stripped of their Belgian citizenship.