Access to Telegram, which the government has accused of being used to foment violence during the protests, was cut in the early afternoon, according to the semi-official ISNA news agency and other media.
Telecoms Minister Mohammad-Javad Azari Jahromi had accused channels on Telegram of being "counter-revolutionary" and encouraging the "use of Molotov cocktails, armed uprising and social unrest".
Iran blocks internet on third night of protests
"If the counter-revolutionaries want to use this atmosphere to create riots, naturally the Supreme National Security Council will take action," he told the state broadcaster.
Telegram CEO Pavel Durov confirmed the blocking.
"Iranian authorities are blocking access to Telegram for the majority of Iranians after our public refusal to shut down [Sedaie Mardom] and other peacefully protesting channels," he tweeted, referring to one of the channels criticised by Iran.
Iranian authorities are blocking access to Telegram for the majority of Iranians after our public refusal to shut down https://t.co/9E4kXZYcP9 and other peacefully protesting channels.
— Pavel Durov (@durov) December 31, 2017
Sedaie Mardom was a replacement for Amadnews, one of the biggest opposition Telegram channels with more than 1.3 million followers, which was removed after Jahromi's complaint on Saturday.
A third night of unrest in Iran saw mass demonstrations across the country, two people killed and dozens arrested.
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