Telegram chief executive Pavel Durov is to appear in court after being arrested at a Paris airport for offences related to his popular messaging app, sources told AFP.
Russia has accused France of "refusing to cooperate" following the arrest of the Franco-Russian billionaire, 39, at Le Bourget airport on Saturday night.
Durov had arrived from Baku, Azerbaijan, and was planning to have dinner in the French capital, a source close to the case said.
An investigating magistrate was to decide later Sunday on a possible extension of Durov's 24-hour detention.
Depending on that decision, he could be charged or released.
France's OFMIN, an office tasked with preventing violence against minors, had issued an arrest warrant for Durov in a preliminary investigation into alleged offences including fraud, drug trafficking, cyberbullying, organised crime and promotion of terrorism, one source said.
Durov is accused of failing to take action to curb the criminal use of his platform.
"Enough of Telegram's impunity," said one investigator who expressed surprise that Durov flew to Paris knowing he was a wanted man.
Russian authorities said they had demanded access to Durov but had no response from France.
"We immediately asked French authorities to explain the reasons for this detention and demanded that his rights be protected and that consular access be granted. Up to now, the French side is refusing to cooperate on this question," Russia's embassy in Paris said in a statement reported by the Ria Novosti news agency.
Russian foreign ministry spokeswoman Maria Zakharova said Moscow had asked for consular access to 39-year-old Durov, saying that as he also had French citizenship "France considers that it is his main nationality".
Businessman Elon Musk posted the hashtag #FreePavel on the X social media platform he owns and commented in French, "Liberte Liberte! Liberte?" (Freedom Freedom! Freedom?).
Former US presidential candidate Robert F. Kennedy Jr said, also on X, that "the need to protect free speech has never been more urgent".
The encrypted messaging app, based in Dubai, has positioned itself as an alternative to US-owned platforms, which have been criticised for their commercial exploitation of users' personal data.
Telegram has committed to never disclosing information about its users.
In a rare interview given to right-wing talk show host Tucker Carlson in April, Durov said he got the idea to launch an encrypted messaging app after coming under pressure from the Russian government while working at VK -- a social network he created before selling it and leaving Russia in 2014.
He said he then tried to settle in Berlin, London, Singapore and San Francisco before choosing Dubai, which he praised for its business environment and "neutrality".
People "love the independence. They also love the privacy, the freedom, (there are) a lot of reasons why somebody would switch to Telegram," Durov told Carlson.
He said at the time that the platform had more than 900 million active users.
By basing itself in the United Arab Emirates, Telegram has shielded itself from moderation laws at a time when Western countries are pressuring large platforms to remove illegal content.
Telegram allows groups of up to 200,000 members, which has led to accusations that it makes it easier for false information to spread virally, as well as for users to disseminate neo-Nazi, paedophilic, conspiratorial and terrorist content.
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