TEHRAN, Iran (AP) — Iranian authorities have ordered internet service providers to block access to Telegram, a popular messaging app used by an estimated 40 million Iranians.

State TV reported Monday that the Tehran prosecutor has ordered that the app be blocked in a way that would not allow users to bypass the restrictions using a virtual private network (VPN).

Iran said last month it would block Telegram, which bills itself as an encrypted message service, for national security reasons. The app was used by the organizers of protests earlier this year, and was temporarily shut down during demonstrations in January.

Iranian authorities say the app is used by terrorists and pornographers.

After Iran announced it would block the app, many government-affiliated users have switched to local alternatives.