"We deny the accusations and forcefully condemn the Iranian diplomat's arrest and call for his immediate release," the foreign affairs ministry said in a statement.

It described the allegations as a conspiracy to "sabotage Iran's ancient and long-standing relations with France and other significant European countries."

The statement was released minutes before a French diplomatic source told AFP that security services believed the Iranian intelligence ministry was behind the foiled plot.

In retaliation, France announced it had frozen assets belonging to two suspected Iranian intelligence operatives as well as Iran's ministry of security and intelligence.

"This extremely serious act envisaged on our territory could not go without a response," France's interior, foreign and economy ministers said in a rare joint statement.

The opposition meeting allegedly targeted was also attended by leading US figures, including close allies of President Donald Trump.

Six people were arrested afterwards in coordinated raids by European police forces, including an Iranian diplomat identified as Assadollah Asadi, who is set to be extradited from Germany to Belgium for prosecution.

Asadi was targeted by France for the six-month asset freeze along with another man named as Saeid Hashemi Moghadam.

Iran has accused the opposition group, the People's Mujahedeen of Iran (MEK), which Tehran considers a "terrorist" group, of orchestrating the plot to discredit President Hassan Rouhani as he embarked on a tour of Europe.