Demonstrators have set themselves on fire to protest against the arrests
French judges have placed 17 members of an Iranian exile group under formal investigation for alleged links to terrorism.
The move - ordered at a special hearing amid tight security in Paris - is the first step towards bringing possible charges against them.
The suspects include Maryam Rajavi, the leader of the political arm of the Iranian People's Mujahideen (MKO), a left-wing organisation designated a terrorist group by the United States and the European Union and by Iran's clerical rulers.
More than 160 suspected MKO members have been arrested by French police in recent days - most were subsequently released.
The 17 were questioned overnight about alleged plans to attack Iranian embassies across Europe.
They are to be investigated for "conspiring with a terrorist organisation", judicial officials said.
Eleven suspects have been remanded in custody - including Maryam Rajavi. They are appealing against their detention.
The other six have been released pending the investigation.
Security around the court was heavy in case supporters of the group tried to set themselves alight - as happened several times last week.
The BBC's Caroline Wyatt in Paris says Maryam Rajavi begged her followers not to harm themselves and most were demonstrating near the opposition group's headquarters in Auvers-Sur-Oise, near Paris, on Saturday.
At least 40 Iranian exiles are in their third day of a hunger strike and say they will continue until Maryam Rajavi and the others are released.
Military wing disarmed
Iran has welcomed France's actions against the group, which seeks the overthrow of the Islamic regime.
Although classified as a terrorist organisation by the US and EU, the MKO maintains offices in several western European cities.
The group's military wing was disarmed by occupying US forces in Iraq in May.
The French Government said it is trying to stop the group from expanding its operations in France.
Interior Minister Nicolas Sarkozy said earlier the MKO "recently wanted to make France its support base, notably after the intervention in Iraq".
"We cannot accept that," he added.
The head of France's domestic intelligence service, Pierre de Bousquet de Florian, said the group was "transforming its Val d'Oise centre [near Paris] ... into an international terrorist base".
Maryam Rajavi is the wife of the group's military leader, Massoud Rajavi. His brother, Saleh Rajavi, has also been placed under investigation in Paris but is among those released.
Ten MKO supporters have set fire to themselves in European cities in protest at the raids at various sites in the north and west of Paris.