Wahaj-us-Siraj, the CEO of Nayatel, a major internet service provider, confirmed that PTA had ordered the ban on YouTube indefinitely.
"They ordered the full YouTube site blocked. So that order was implemented by the internet service providers and internet backbone providers," Siraj said.
He told the AFP news agency that blocking Facebook and YouTube would slash up to a quarter of all internet traffic in Pakistan.
"It really is very hard to see how there's going to be any easy outcome to this. And I feel we're in for a long haul dispute where a lot of people are going to have to learn to compromise"
Barry Fox, London-based technology analyst
"These two sites take 20 to 25 per cent of the country's total internet traffic so we are seeing a drop in internet traffic," he said.
The PTA had blocked Facebook on Wednesday obeying a Pakistani court decision to ban the site following complaints over a user-generated contest page that encourged members to post caricatures of Prophet Mohammed.
Any images or pictures that depict any of Islam's prophets or messengers are deemed forbidden and blasphemous by Islamic law.
Barry Fox, a technology analyst in London, said the "move can only be a temporary measure".
"We're in this for a long haul because you have a basic conflict here," Fox told Al Jazeera.
"Internet is about free traffic of information round the world without barriers, and some country governments and religious leaders are gainst the free traffic of all information.
"So it really is very hard to see how there's going to be any easy outcome to this. And I feel we're in for a long-haul dispute where a lot of people are going to have to learn to compromise."
But Fox said people who are "really tech-savvy" would still be able to get round the barriers created by the government.
"They'll use ... proxy servers but what this means is that their system redirects barriers round other countries," he said.
"So people, if want they want to access banned sites, they'll be able to do so. However, they put themselves at the risk of detection by what is the equivalent in China of the internet police who, when they find people getting round these barriers, they are liable to be prosecuted - and that's the big risk."
Al Jazeera's Kamal Hyder, reporting from Islamabad, Pakistan's capital, said: "It [sacrilegious content] has disturbed and infuriated a lot of people in the deeply conservative country.
"And so now many people are saying that they will block Facebook completely.
"The court has said that this (ban) will continue until the 31st of May. However, after the 31st there will be a decision.
"But there is already a growing number of people who are saying it (Facebook) should be banned completely.
"Hundreds of people have been protesting and it is likely to continue for a few more days."
Pakistan blocked Youtube in 2007 for about a year in a similar protest against "blasphemous" videos.