The move by the PTA follows its block of Facebook on Wednesday.
In a statement on Thursday the government said it was blocking YouTube and Facebook after trying all possible avenues to get the sites to remove material it considered derogatory.
The PTA did not describe the content that was found sacrilegious and derogatory. It said it was acting on the orders of the high court and the Pakistani government.
A Pakistani court ordered on Wednesday that Facebook should be blocked because of a page inviting people to draw caricatures of the Prophet Mohammed.
YouTube said it had received reports of PTA’s orders to Internet service providers in Pakistan to block access to its site. “We are looking into the matter and are working to ensure that the service is restored as soon as possible,” Scott Rubin, a YouTube spokesman, said in an e-mail. Content that violates its guidelines is removed as soon as YouTube becomes aware of it, he added.
Facebook and YouTube did not immediately respond to requests for comment.
The information provided on the Facebook page, which had more than 84,000 fans on Thursday, said that the aim of the page was not to slander Muslims. The objective of the site was to show extremists, some of whom threaten to harm people because of their depictions of the Prophet Mohammed, that they are not afraid of them, it added.
The PTA said that on May 18 it Instructed operators and others concerned to block the objectionable URL on Facebook.
The PTA said that it has so far blocked more than 450 links on the Internet containing derogatory material. Some media stories are reporting that some links to Wikipedia have also been blocked.
The PTA has said it would welcome the authorities at Facebook and YouTube to contact the agency to resolve the issue. The regulator has meanwhile provided a toll-free number and e-mail address where people can complain about URLs that link to objectionable material.