A Russian court has ordered that access to the Telegram messenger service should be blocked, according to the state-run news agency TASS.
The app is very popular across Russia as well as many countries in the Middle East.
Last week Russia's state communications watchdog filed a lawsuit to limit access to the app after the company refused to give Russian state security services access to its users' secret messages.
The FSB agency says it wants access so it can read messages and prevent future terror attacks in the country.
Telegram failed to meet an April 4 deadline to hand over the encryption keys. The company said the way the service was built means it has no access to customers' keys.
TASS reported that on Friday the Russian government started to enforce the court ruling.
Telegram's lawyers say the official attempt to stop the app being used in Russia is "groundless".