On April 13, Tagansky Court considered the suit of Roskomnadzor and ruled to immediately block Telegram since the messenger refused to hand over its encryption keys to the special services, as required by the Russian law.
Interfax news agency quoted an official at the watchdog as saying it would take several hours to complete the operation to block access.
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.
The Federal Service ordered telecommunications carriers to limit access to the messenger pursuant to the April 13 decision of the Tagansky Court.
The FSB has said it needs such access to guard against security threats such as terrorist attacks.
"There is a certain legislation that demands certain data to be passed to certain services of the Russian Federation", Kremlin spokesperson, Dmitri S. Peskov, said, adding that his office would resort to an alternative app the moment Telegram ceased working. Unfortunately it seems that it is this level of security and privacy that has resulted in the app getting banned in Russian Federation.
The FSB has claimed that worldwide terrorist groups actively use Telegram's secret chats with high levels of encryption.
"We consider the ban decision anti-constitutional and will continue to defend the right to secret correspondence for Russians". The company refused to provide the information, and the service founder, Pavel Durov, stated that the company would not comply with laws that violate privacy of its users and contradict the messenger's values.
Telegram is widely used in countries across the former Soviet Union and Middle East.
President Vladimir Putin's press office reportedly uses the Telegram as well.