Russia's state telecommunications regulator said on Monday it had begun blocking access to Telegram messenger after the company refused to comply with an order to give Russian state security access to its users' secret messages.
The watchdog, Roskomnadzor, said in a statement on its website that it had sent telecoms operators a notification about blocking access to Telegram inside Russia.
The service, set up by a Russian entrepreneur, has more than 200 million global users and is ranked as the world's ninth most popular mobile messaging app.
A Roskomnadzor official said it would take several hours to complete the operation to block access, Interfax news agency said.
In Moscow, the Telegram app was still functioning as normal by mid-afternoon on Monday, but the company's website had been blocked by two of Russia's biggest service providers, MTS and Megafon.
Both MTS and Megafon declined to comment.
Roskomnadzor was implementing a decision handed down on Friday by a Russian court, which ruled that Telegram should be blocked because it was in violation of Russian regulations.
Telegram has repeatedly refused to comply with requests to give Russia's Federal Security Service (FSB) access to its users' encrypted messages.
The FSB has said it needs such access to guard against security threats such as terrorist attacks. But Telegram said compliance would violate users' privacy.
Telegram's founder and CEO, Pavel Durov, said the ban would damage the quality of lives of 15 million Russians and do nothing to improve Russia's security.
"The terrorist threat in Russia will stay at the same level, because extremists will continue to use encrypted communication channels - in other messengers, or through a VPN," he said.
"We consider the ban decision anti-constitutional and will continue to defend the right to secret correspondence for Russians."
Durov was a pioneer of social media in Russia but left the country in 2014. He has since been a vocal critic of the Kremlin's policies on Internet freedom.
Telegram is widely used in countries across the former Soviet Union and Middle East.
As well as being popular with journalists and members of Russia's political opposition, Telegram is also used by the Kremlin to communicate with reporters and arrange regular conference calls with President Vladimir Putin's spokesman.
On Monday, the spokesman's office asked journalists who were previously subscribed to a chat in Telegram to switch to a chat that had been set up in a different messaging service, ICQ, which is part of the Russian Mail.ru technology group .