An Iranian diplomat was among three people arrested on suspicion of plotting a bomb attack on a meeting of an exiled Iranian opposition group in France attended by Donald Trump's lawyer Rudy Giuliani, Belgian authorities say.

Key points: Two of those arrested are charged with terrorism offences

Two of those arrested are charged with terrorism offences Attack allegedly planned against meeting of umbrella bloc of Iranian opposition parties

Attack allegedly planned against meeting of umbrella bloc of Iranian opposition parties Iran's Foreign minister dismissed diplomat's arrest as "false flag ploy"

Three other arrests were made in France but two of those people were later released.

Several former European and Arab ministers also attended the meeting of the Paris-based National Council of Resistance of Iran (NCRI) — an umbrella bloc of opposition groups in exile that seek an end to Shiite Muslim clerical rule in Iran.

Iran's Foreign Minister called the news a sinister "false flag ploy" and said Tehran was ready to work with all concerned parties to get to the bottom of it.

Two suspects in Belgium were intercepted by Belgian police on Saturday, with 500 grams of TATP, a home-made explosive produced from easily available chemicals, as well as a detonation device found in their car, a joint statement by the Belgian prosecutor and the intelligence services said.

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The 38-year-old man and a 33-year-old woman, identified only as Amir S and Nasimeh N, were charged with attempted terrorist murder and preparation of a terrorist act, it said.

The diplomat at the Iranian embassy in the Austrian capital Vienna was arrested in Germany, the Belgian statement said.

Iranian President Hassan Rouhani is due to visit Austria on Wednesday.

Three people of Iranian origin were arrested in France to assess their link to the Brussels' suspects, a French judicial source said.

Two were released due to a lack of evidence against them while the third was held for questioning, the source said.

"How convenient: Just as we embark on a presidential visit to Europe, an alleged Iranian operation and its 'plotters' arrested," Foreign Minister Mohammad Javad Zarif tweeted.

"Iran unequivocally condemns all violence & terror anywhere, and is ready to work with all concerned to uncover what is a sinister false flag ploy."

Donald Trump's lawyer Rudy Giuliani was at the NCRI meeting. ( Reuters: Regis Duvignau )

Thousands attend NCRI meeting

The Belgian statement gave no further details about the diplomat, other than saying they were suspected of having been in contact with the Belgian pair arrested.

It said one of those arrested in France, Merhad A, was a suspected accomplice of the pair in Brussels.

The NCRI meeting, which attracted a crowd of thousands, took place on Saturday in Villepinte, just outside Paris.

"A plot by the religious dictatorship ruling Iran to carry out a terrorist attack against the grand gathering of the Iranian Resistance in Villepinte was foiled," Shahin Gobadi, a spokesman for NCRI said.

The People's Mujahideen Organisation of Iran is the main component of NCRI.

The group, also known by its Persian name Mujahideen-e-Khalq, was once listed as a terrorist organisation by the United States and the European Union but is no longer.

Tehran has long called for a crackdown on the NCRI in Paris, Riyadh and Washington.

Reuters