Telegram says arrested CEO Durov has 'nothing to hide'.
Pavel Durov delivers a keynote address at the Mobile World Congress in Barcelona, Spain in 2016.
The investigation is reportedly about insufficient moderation, with Mr Durov accused of failing to take steps to curb criminal uses of Telegram.
Mr Durov was arrested at an airport north of Paris under a warrant for offences related to the app, according to officials.
Messaging app Telegram has said its CEO Pavel Durov, who was detained in France on Saturday, has "nothing to hide".
"Almost a billion users globally use Telegram as means of communication and as a source of vital information," the app's statement read.
Pavel Durov, 39, was born in Russia and now lives in Dubai, where Telegram is based. »