A case was opened last month to investigate child pornography, drug sales, fraud and other criminal activities on the platform. The app’s founder, Pavel Durov, was detained over the weekend near Paris.

Prosecutors in France said on Monday that Pavel Durov, the entrepreneur who runs the Telegram messaging platform, had been arrested in connection with an investigation opened last month into criminal activity on the platform and a lack of cooperation with law enforcement.

Mr. Durov, 39, was detained on Saturday at Le Bourget Airport near Paris after landing on a private plane from Azerbaijan. He has not been charged and remains in custody, which can be extended through Wednesday, according to prosecutors.

Laure Beccuau, the Paris prosecutor, said in a statement that the arrest was part of an investigation opened on July 8 “against person unnamed” on a raft of potential charges, including complicity in the distribution of child pornography and selling of drugs, money laundering, and a refusal to cooperate with law enforcement.

The investigation is being handled by cybercrime and anti-fraud specialists, Ms. Beccuau said. “It is within this procedural framework that Pavel Durov was questioned by the investigators,” she said.

It is unclear whether any of the charges listed by Ms. Beccuau will be held against Mr. Durov.

In France, complex criminal cases are handled by special magistrates who have broad investigative powers and can place defendants under formal investigation, charging them when they believe the evidence warrants it.

But the magistrates can later drop charges if they do not think evidence is sufficient to proceed to trial, and cases can take years.

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