Access to the social network Facebook has also been barred as part of a crackdown on websites seen to be hosting un-Islamic content.
On Wednesday a Pakistani court ordered Facebook to be blocked temporarily in a row over a competition involving caricatures of the Prophet Muhammad.
Some Wikipedia pages are also now being restricted, latest reports say.
There has been no response yet from YouTube. The site was briefly blocked in Pakistan in 2008 – ostensibly for carrying material deemed offensive to Muslims.
Correspondents say it remains to be seen how successful the new bans will be in Pakistan and whether citizens find a way round them.
Facebook said on Wednesday it was investigating the competition on its pages but that the content did not violate its terms.
There have been protests in several Pakistani cities against the Facebook competition.
The Pakistan Telecommunications Authority authorities said it had ordered internet service providers to “completely shut down” YouTube and Facebook from being viewed within Pakistan.
It said the move came only after “all possible avenues” within its jurisdiction had been used.
These, it said, included “using regular channels available on Facebook and YouTube to launch [a] protest, to avoid appearances of derogatory material available on their websites”.
Such material, the PTA said, had “increased in numbers as time passed by”.
“PTA has so far blocked more than 450 links on the internet containing derogatory material,” the statement said, adding that its move was an extension of Wednesday’s high court order.
Correspondents say it is not clear how far the authorities ban might extend – the order passed in Lahore only concerned Facebook.
Facebook said in a statement on Wednesday: “While the content does not violate our terms, we do understand it may not be legal in some countries.
“In cases like this, the approach is sometimes to restrict certain content from being shown in specific countries.”