PARIS—Since an Islamist radical beheaded a French schoolteacher this month, authorities have detained dozens of suspected extremists, moved to disband Muslim civic groups and shut down a mosque where thousands worship, escalating a long-simmering conflict over the place of Islam in French society.
Those measures are part of a campaign by the government of President Emmanuel Macron to counter what it calls “Islamic separatism,” which senior officials describe as the dominance of religious rules over state authority in some…