Published: May 20, 2010 10:54 AM EDT

ISLAMABAD (AP) - The Pakistani government blocked access to YouTube on Thursday because of "sacrilegious" content on the video-sharing website, signaling a growing Internet crackdown against sites deemed offensive to the country's majority Muslim population.

The move against YouTube came a day after the government blocked access to Facebook amid anger over a page on the social networking site that encourages users to post images of Islam's Prophet Muhammad. The page sparked criticism because Islam prohibits any images of the prophet.

The Pakistan Telecommunications Authority did not point to specific material on YouTube that prompted it to block the site, only citing "growing sacrilegious contents."

The government took action against both Facebook and YouTube after it failed to persuade the websites to remove the "derogatory material," the regulatory body said in a statement. It welcomed representatives from the two websites to contact the Pakistani government to resolve the dispute in a way that "ensures religious harmony and respect."

The regulatory body said it has blocked more than 450 Internet links containing offensive material, but it is unclear how many of the links were blocked in the last two days.

Access to the online encyclopedia site Wikipedia and the photo sharing site Flickr also was restricted Thursday. The government blocked Facebook on Wednesday after a group of Islamic lawyers won a court order requiring officials to restrict access to the site until May 31. It was unclear if the ban against YouTube would also be temporary.

The webpage at the center of the Facebook dispute - "Everybody Draw Mohammed Day!" - encourages users to post images of the prophet on May 20 to protest threats made by a radical Muslim group against the creators of "South Park" for depicting Muhammad in a bear suit during an episode earlier this year.

The page sparked protests from radical students in Pakistan, with some holding signs urging Islamic holy war against those who blaspheme the prophet. A series of cartoons of the prophet published in a Danish newspaper in 2005 sparked violent protests by Muslims around the world, including in Pakistan, and death threats against the cartoonists.

Facebook said Wednesday it was investigating the controversial page.

"While the content does not violate our terms, we do understand it may not be legal in some countries," the company said in a statement. "In cases like this, the approach is sometimes to restrict certain content from being shown in specific countries."

Online reaction to the Facebook ban was supportive in the initial hours after it was implemented. But comments on Twitter -- which was still unblocked Thursday and drawing new users thanks to bans on other sites -- showed many internet users were angry about the new, wide-ranging restrictions. "Sad and embarrassing day in the history of Pakistan. Tough times to be a Pakistani. Questionable decisions in a so called 'democracy,"' one user tweeted.

Pakistan blocked access to YouTube once before for two days in 2008 because of what it said was unIslamic content. Turkey, Thailand, Indonesia and Morocco have all blocked access to YouTube in the past for various reasons.

It remains to be seen how successful the government will be at keeping Pakistan's nearly 20 million Internet users from accessing the blocked sites.

Other countries, such as China, permanently ban Facebook and YouTube. But citizens often have little trouble working their way around the ban using proxy servers and other means. "What's common to Facebook and Lashkar-e-Taiba?" one user on Twitter wrote, referring to a Pakistani militant group. "They are both banned in Pakistan, but Pakistanis can still find them if they want to."