Most victims are of Asian, African or Middle Eastern descent and belong to France's Muslim community, the largest in Europe.
While countries such as Britain have set up special units that track down victims at home and overseas, activists say France is only now waking up to the problem.
"For a long time this used to be considered a cultural thing," Fatima Lalem, who is in charge of gender equality at Paris City Hall, told Reuters. "Something that happens, but that people don't look at too closely." Over the past year, France has begun to tackle the problem more aggressively. Last November, Paris City Hall published a guide advising officials on detecting forced marriages.
But former victims and activists, many of them second- or third-generation immigrants working in France's multicultural suburbs, said such moves were unlikely to help women married off abroad, or scared into silence.
Zeliha Alkis, who works for Elele, a non-profit organization that mainly helps women of Turkish origin, cites the example of a young woman of Turkish descent who was married to a Turkish man at a Paris town hall this summer. On her wedding night, she was locked in a room, and when she screamed and protested, her grandmother tied her up so the marriage could be consummated.
In many other cases, the women are married in the family's country of origin. Reasons include the family wanting to ensure the woman marries a candidate deemed suitable, settling a debt and procuring a visa for the groom. Unlike arranged marriage, which can be consensual, these weddings are performed against the women's will.
"Early June to late September is our busiest period, after the wedding was celebrated over there, and then the young woman calls us and says, what do I do now, I don't want that husband," said Alkis, who has received death threats from angry families.
No one knows exactly how many French women are forced into such unions. A report on women's rights by France's high council for integration in 2003 put the number of girls and women at risk of being forced into marriage at 70,000, based on research by grassroots organizations.
Christine Jamaa, head of non-profit organization Voix de Femmes, and her team help more than 200 women a year flee forced marriages or escape before the wedding.
Since more than half of them are under the age of 20 and at school or college, Jamaa is asking the government to set up an early warning system for those who disappear over the holidays.
"There are those who don't come back from the summer holidays and sometimes the schools only react in October, even in February," Jamaa told Reuters at her office north of Paris.
France has at times steered an awkward course between trying to respect its ethnic minorities and defending women's rights; last year, a French court annulled the marriage of a Muslim woman on the grounds that she lied about being a virgin.
Similarly, some see forced marriage as a religious issue, even though Islam bans it and Imams helping the women have been threatened.
In the families' countries of origin, forced marriage tends to be illegal and less widespread than in migrant communities.
For now, activists rely on friends, siblings, teachers or the victims themselves to alert them. Sympathetic French consulates abroad may then intervene to delay a husband's visa.
One such consulate assisted the escape of Fatou Diouf, a 28-year-old Muslim woman of Senegalese descent.
She was lured abroad at the age of 18 - her father told her he had won a trip to Senegal and gave her the ticket. When she arrived, she was told she had already been married off to her 36-year-old uncle in a religious ceremony that did not require the spouses to be present.
After 7 months in confinement, she fled with the help of a local who put her in touch with the French consulate. Now, she works for a women's organization, Femmes Solidaires, in a tiny office in the shadow of grimy tower blocks south of Paris.
"There are more and more girls who are speaking out," she told Reuters. "But exercising the right to say no automatically means cutting the ties with the family."
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