They said the 15 were seized in raids ordered by anti-terrorism judges in at least two suburbs and some were suspected of having links with Moroccan groups.
"Fifteen people have been detained, nine men and six women," said an Interior Ministry spokesman, adding that they were detained in Aulnay-sous-Bois, a suburb east of Paris, and Mantes-la-Jolie west of the capital.
The judicial sources made no direct link with the Madrid train bombings which killed 191 people last month.
But one of the sources said: "Some of the people detained are suspected of having links with Moroccan Islamic groups which Spanish investigators say might have been involved in the Madrid bombings."
The suspects can be held in detention for up to 96 hours without being formally placed under investigation - the final step under French law before formal charges can be pressed.
Forty-five people were killed, including the bombers, in almost simultaneous attacks in Casablanca on 16 May last year. The sources said the raids were ordered as part of an investigation into the death of a Frenchman in the attacks.
France is on high alert after the Madrid train bombings. A railway worker found a bomb half-buried on the main train line between Paris and Switzerland on 24 March.
The French authorities have also received threats in letters sent by a self-proclaimed "Islamic" group whose authenticity is in doubt.
More than 100 people were killed
in the Madrid train blasts
Another group demanding cash said it had planted bombs on the French railway before suspending its threats last month. The group, calling itself AZF, said it would improve its methods and then could stage an attack more deadly than the Madrid bombings.
French police have also arrested several suspected leaders of the Basque separatist group ETA in recent weeks. They found what they believe was an ETA arms factory and a large supply of weapons on a farm in southwestern France on Sunday.