Telegram chief executive Pavel Durov has been arrested by French police at an airport north of Paris.
Mr Durov was detained after his private jet had landed at Le Bourget Airport, French media reported.
According to officials the 39-year-old billionaire was arrested under a warrant for offences related to the popular messaging app. The investigation is reportedly about a lack of moderation, with Mr Durov accused of failing to take steps to curb criminal uses of Telegram.
The app is accused of failure to cooperate with law enforcement over drug trafficking, child sexual content and fraud. Telegram has previously denied having insufficient moderation.
Pavel Durov was born in Russia and now lives in Dubai, where Telegram is based. He holds dual citizenship of the United Arab Emirates and France.
Telegram is particularly popular in Russia, Ukraine and former Soviet Union states.
The app was banned in Russia in 2018, after a previous refusal by him to hand over user data. The ban was reversed in 2021.
Telegram is ranked as one of the major social media platforms after Facebook, YouTube, WhatsApp, Instagram, TikTok and Wechat.
Mr Durov founded Telegram in 2013. He left Russia in 2014 after refusing to comply with government demands to shut down opposition communities on his VKontakte social media platform, which he sold.
On Sunday, the Russian Embassy in France wrote on Facebook that it was seeking to “clarify the reasons for the detention and to provide for the protection of Mr Durov’s rights and facilitate consular access”.
The post added that French authorities had not been cooperating with Russian officials.
Russian Foreign ministry spokeswoman Maria Zakharova posted on Telegram asking whether Western human rights NGOs would be silent on Mr Durov’s arrest, after they criticised Russia’s decision to “create obstacles” to the work of Telegram in Russia in 2018.
Several Russian officials condemned the businessman’s arrest, saying it showed the West has double standards when it comes to free speech and democracy.
American whistleblower Edward Snowden, who has been living in exile in Russia since 2013, said on X that Mr Durov’s arrest “was an assault on the basic human rights of speech and association”.
X owner Elon Musk, who has faced extensive criticism over moderation and material hosted by his own social media site, posted repeatedly about the situation.
He hashtagged one post #freepavel, and in another wrote: “POV [Point of view]: It’s 2030 in Europe and you’re being executed for liking a meme.”
Telegram allows groups of up to 200,000 members, which critics have argued makes it easier for misinformation to spread, and for users to share conspiracist, neo-Nazi, paedophilic, or terror-related content.
In the UK, the app was scrutinised for hosting far-right channels that were instrumental in organising the violent disorder in English cities earlier this month.
Telegram did remove some groups, but overall its system of moderating extremist and illegal content is significantly weaker than that of other social media companies and messenger apps, say cybersecurity experts.
Correction: An earlier version of this story misidentified Pavel Durov’s lawyer. It has been updated to remove the reference.