Billionaire Telegram founder Pavel Durov was arrested after arriving in France on his private jet at the weekend.
He was reportedly the subject of a search warrant, with French news outlets suggesting an investigation focused on a lack of moderators on the encrypted messaging app and potential criminal activity by users.
Telegram is one of the most downloaded apps in the world.
But who is the man behind it - and how does the app work?
'Russia's Mark Zuckerberg'
Born in Russia, Pavel Durov is the founder and owner of Telegram - a free-to-use messaging app that competes with other platforms such as WhatsApp, Facebook Messenger and TikTok.
His fortune is currently estimated by Forbes at $15.5bn (£11.7bn) - making him the 120th richest person in the world - and he is sometimes dubbed "Russia's Mark Zuckerberg".
The 39-year-old left the country in 2014, a year after Telegram was launched, after refusing to comply with government demands to shut down opposition communities on VKontakte - an earlier social media platform he had founded with his brother, and later sold.
He moved to Dubai in 2017 and became a French citizen in August 2021.
According to media reports, Durov has also received United Arab Emirates citizenship, and is also a citizen of St Kitts and Nevis, a dual-island nation in the Caribbean.
Telegram's popularity has led to rise in scrutiny
The Telegram app is influential in Russia, Ukraine and former Soviet republics.
However, Russia began blocking the app in 2018 after the company refused to comply with a court order to grant state security services access to its users' encrypted messages. The action had little effect on the availability of Telegram there, but did spark mass protests in Moscow and criticism from non-governmental organisations (NGOs).
The app is becoming increasingly popular, and is now among the top five most downloaded around the world, the company says. In 2023, it passed 700m active monthly users - and the aim is to surpass 1bn within a year.
However, its growing popularity has led to increased scrutiny from several countries in Europe, including France, on potential security and data breach concerns.
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"Telegram is committed to protecting user privacy and human rights such as freedom of speech and assembly," the company says on its website. "It has played a prominent role in pro-democracy movements around the world, including in Iran, Russia, Belarus, Myanmar and Hong Kong."
Durov is also reportedly a sperm donor who has helped more than 100 couples in 12 countries to have children.
He is known for often dressing in black clothing, in a look similar to the character Neo from the sci-fi Matrix film series.
Following his arrest, several activists have gathered to show their support in Moscow. Paper planes, referring to Telegram, have been placed near the French embassy in Moscow.
Speaking about leaving Russia to controversial US media personality Tucker Carlson in April, Durov said he would "rather be free than to take orders from anyone".