French police have launched a massive raid in the Paris region on the Iranian opposition group, the People's Mujahideen.
More than 150 people are reported to have been arrested during the operation by 1,300 police officers, on the orders of the anti-terrorist judge Jean-Louis Bruguiere.
It follows a long running investigation into the activities of the People's Mujahideen, a group which has in the past been declared a terrorist organisation by Iran, the United States and the European Union.
BBC correspondent Caroline Wyatt, in Paris, said police sources said the operation targeted some 150 people who they believe were "preparing to commit or finance acts of terrorism".
French news agency AFP said police had arrested Maryam Rajavi, wife of the group's leader, Massoud Rajavi.
She is seen by the People's Mujahideen as the "future president of Iran".
Her brother, Saleh Rajavi, is also reported to have been arrested when police stormed the European headquarters of the National Council of Resistance of Iran (NCRI), Iran's main political
opposition umbrella group.
A large quantity of money and computer equipment is also reported to have been seized in the raids in the Val d'Oise region north
of Paris and in the Yvelines region to the west.
Spokesman for the People's Mujahideen, Ali Safavi, told AFP the "allegations are absolutely preposterous".
He said People's Mujahideen personnel had been living in France for many years "and there has been no problem.
"Wherever they are, they are absolutely not involved in illegal activities in their host country."
He accused France of "trying to curry favour with the Islamic government in Iran" and said the arrests were part of a "concerted conspiracy" between the two governments.
The EU declared the People's Mujahideen a terrorist organisation in May last year, but this is the first time that the French authorities have moved to detain its members.