"In order to maintain public order and safety, access to certain social media applications has been restricted temporarily," a senior telecommunications authority official told Reuters, without specifying which social media.
The interior ministry said in a statement the block would last until 1500 local time (1000 GMT) and applied to YouTube, Facebook, WhatsApp, Twitter, Telegram and Tiktok.
Pakistan said this week it had banned the Islamist group Tehrik-i-Labaik Pakistan (TLP) after the arrest of its leader sparked major nationwide protests that were also fuelled by anti-France sentiment.
A second security source said that the block was linked to an operation underway against the group.
"As the government announced earlier...wherever we need we will be blocking social media to crack down against Tehrik-i-Labaiak," he said, on condition of anonymity as he was not authorized to speak to media.
The country has been mired in protests by TLP this week. At least four policemen have been killed in three days of violence, according to the country's information minister. Nearly 600 people have been wounded, with 200 in critical condition,
The group has been agitating against the arrest of its chief ahead of countrywide rallies to denounce the publication of cartoons in France depicting the Prophet Mohammad.
For Muslims, depictions of the Prophet are blasphemous.
Some rights activists criticized Friday's social media blackout, warning it could lead to more severe curbs on freedoms.
"These arbitrary decisions of blocking and banning have never done any good (and) instead opened ways to blanket bans," said Nighat Dad, head of the Digital Rights Foundation on Twitter, shortly before the site became inaccessible.