Iran on Tuesday denied French accusations that one of its diplomats was involved in an alleged bomb plot targeting an exiled opposition group near Paris in June.
"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 news agency 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 (or MEK), which Tehran considers a "terrorist" group, of orchestrating the plot to discredit President Hassan Rouhani as he embarked on a tour of Europe.
The counter measures by France could have major diplomatic repercussions at a time when France is working to keep Iran in the 2015 accord to limit its nuclear programme amid major tensions in the Middle East.
French President Emmanuel Macron had vowed to visit Tehran early in his term as president, but major differences between the countries on issues such as the wars in Syria and Yemen, as well as the bomb plot, have led to tensions.
Raids on Shia Muslim centres
Also on Tuesday, around 200 police launched a dawn anti-terror raid on one of the biggest Shia Muslim centres in France, the Zahra Centre France, as well as the homes of its directors.
A total of 11 people were questioned and three were arrested, security sources told AFP, including for the illegal possession of firearms.
The Zahra Centre France was founded in 2009 by Yahia Gouasmi, who has spoken in support of Lebanese group Hezbollah and the Iranian supreme leader Ayatollah Ali Khamenei.
Gouasmi is also the founder of the Anti-Zionist Party in France.
The raids on Tuesday were staged as part of the "prevention of terrorism" procedures, regional authorities said in a statement.