Brussels (AFP) - Belgian prosecutors on Monday charged a husband and wife over a plot to bomb a weekend rally by an exiled Iranian opposition group in France.

Amir S. and Nasimeh N., both Belgian nationals, "are suspected of having attempted to carry out a bomb attack" on Saturday in the Paris suburb of Villepinte, during a conference organised by the People's Mujahedin of Iran, a statement from the Belgian federal prosecutor said.

The couple, described by prosecutors as being "of Iranian origin", carried 500 grams of the volatile explosive TATP along with a detonation device when an elite police squad stopped them in a residential district of Brussels.

Belgian prosecutors said an alleged accomplice was under arrest in France, while two others were released after questioning by French police.

The statement said that an Iranian diplomat at the Austrian embassy in Vienna, a contact of the couple, was also arrested in Germany.

Police carried out five raids across Belgium on Saturday linked to the affair, authorities said, though they refused to detail any results of the operation.

The statement said about 25,000 people attended the rally in France.

The People's Mojahedin (MEK), formed in the 1960s to overthrow the shah of Iran, fought the rise of the mullahs in Tehran following the 1979 Islamic revolution.

It earned itself a listing as a "terrorist organisation" by the US State Department in 1997 and was only removed from terror watchlists by the European Union in 2008 and Washington in 2012.

Belgium has been on high alert since the smashing of a terror cell in the town of Verviers in January 2015 that was planning an attack on police.

Belgium further raised its terror alert level after the Paris attacks in November that year, and placed the capital Brussels on lockdown for a week.

Belgium was then hit by its own IS suicide attacks on Brussels airport and a metro station, which killed 32 people in March 2016. Both bombings used TATP explosives.