An Iranian diplomat on trial in Belgium for allegedly plotting to bomb an Iranian opposition rally outside Paris has refused to appear in the dock, his lawyer said.

Assadollah Assadi, formerly based at the Iranian embassy in Austria, is claiming "diplomatic immunity" his lawyer Dimitri de Beco told reporters outside the court in Antwerp, adding that he would be representing his client.

The case shines another uncomfortable light on Iran's international activities just as it hopes to ease tensions with the United States after President Donald Trump tore up the 2015 nuclear deal signed by both countries and other world powers.

It also comes a day after a prisoner swap that saw the release of three Iranians jailed over a 2012 bomb plot in Thailand, in exchange for the freeing of an Australian-British lecturer imprisoned by Tehran for alleged spying.

In June 2018, Belgian authorities thwarted what they said was an attempt to smuggle explosives to France to attack a meeting of one of Iran's exiled opposition movements.

Later that year, the French government accused Iran's intelligence service of being behind the operation, a charge the Islamic republic has furiously denied.

Lawyers of National Council of Resistance of Iran William Bourdon and Georges-Henri Beauthier (R) answer journalists' questions as they arrive at Antwerp courthouse

Assadollah Assadi, a 48-year-old Iranian diplomat formerly based in Vienna, faces life in prison if convicted.

The National Council of Resistance in Iran (NCRI), which includes the People's Mojahedin of Iran or (MEK), organised a rally in Villepinte outside Paris on 30 June 2018.

Several well-known international figures - including former US and British officials and Franco-Colombian former senator Ingrid Betancourt - and NCRI leader Maryam Rajavi were to attend.

On the same morning, Belgian police intercepted a Belgian-Iranian couple driving from Antwerp and carrying half-a-kilo of TATP explosives and a detonator.

The arrested couple, 36-year-old Nassimeh Naami and 40-year-old Amir Saadouni, join Mr Assadi in the dock, alongside another alleged accomplice, Mehrdad Arefani, 57.

All four are charged with attempting to carry out a terrorist attack and taking part in the activity of a terrorist group. All face life sentences.

Mr Assadi was arrested while he was travelling through Germany where he had no immunity from prosecution, being outside of the country of his diplomatic posting.

Mr Arefani, an Iranian poet who had lived in Belgium for more than a decade, was arrested in France in 2018 after Belgium issued a European arrest warrant.

Dimitri de Beco, lawyer of Iranian diplomat Assadollah Assadi, arrives at Antwerp courthouse

Counsel representing those targeted by the alleged attack say Mr Arefani was close to Mr Assadi, said to be the architect of the plot, and point to an Austrian SIM card found in his possession.

The two men deny any connection. "We are looking at a clear case of state terrorism," said lawyer Georges-Henri Beauthier, who is representing the interests of the NCRI, along with French colleague William Bourdon.

Dimitri de Beco, defence counsel for Mr Assadi, has accused the civil plaintiffs of trying to turn the case into a political trial on behalf of the opposition movement.

According to Iran expert Francois Nicoullaud - a former French ambassador to Tehran - Iran's President Hassan Rouhani was surprised to learn about the failed attack.

"Visiting Europe at the time, he was absolutely furious to learn about this intelligence service operation, on which he hadn't been consulted," the diplomat said.

At the time of the alleged plot, Mr Rouhani was trying to maintain the support of European capitals for the 2015 Iran nuclear deal abandoned by the Trump administration.

When Paris pointed the finger at Iranian intelligence, an Iranian spokesman voiced denial and alleged that opponents of the deal in "certain quarters" were attempting to frame Tehran.

That idea was dismissed by observers like Mr Nicoullaud as a smokescreen. "It's not serious," he said.

The trial is scheduled to take two days, today and next Thursday.

The court is then expected to adjourn to consider its verdict before ruling early next year.