Pakistan has blocked the popular video sharing website YouTube because of its "growing sacrilegious content".
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 because of a page inviting people to draw images of the Prophet Muhammad.
Some Wikipedia pages are also now being restricted, latest reports say.
Correspondents say it remains to be seen how successful the new bans will be in Pakistan and whether citizens find a way round them.
YouTube says it is "looking into the matter and working to ensure that the service is restored as soon as possible". The site was briefly blocked in Pakistan in 2008 - ostensibly for carrying material deemed offensive to Muslims.
Facebook said on Wednesday that the content did not violate its terms.
There have been protests in several Pakistani cities against the Facebook competition.
The Pakistan Telecommunications Authority said it had ordered internet service providers to "completely shut down" YouTube and prevent Facebook from being viewed within Pakistan.
It said the move came only after "all possible avenues" within its jurisdiction had been used.
"Before shutting down (YouTube), we did try just to block particular URLs or links, and access to 450 links on the internet were stopped," said PTA spokesman Khurram Ali Mehran.
"But the blasphemous content kept appearing so we ordered a total shut down."
One of the links blocked is to a BBC News website article about Pakistani soldiers apparently beating Taliban suspects in a video posted on Facebook.
A YouTube spokesperson said: "YouTube offers citizens the world over a vital window on cultures and societies and we believe people should not be denied access to information via video.
"Because YouTube is a platform for free expression of all sorts, we take great care when we enforce our policies. Content that violates our guidelines is removed as soon as we become aware of it."
The controversy began with the Facebook feature called "Everybody Draw Muhammad Day". Depictions of the Prophet are forbidden in Islam.
A message on the item's information page said it was not "trying to slander the average Muslim".
"We simply want to show the extremists that threaten to harm people because of their Muhammad depictions that we're not afraid of them."
The page contains drawings and caricatures of the Prophet Muhammad and characters from other religions, including Hinduism and Christianity.
"Such malicious and insulting attacks hurt the sentiments of Muslims around the world and cannot be accepted under the garb of freedom of expression," Pakistani foreign ministry spokesman Abdul Basit said about the page.
Facebook said in a statement that it would take action if any content "becomes an attack on anyone, including Muslim people", but that in this case its policies were not violated.
"Facebook values free speech and enables people to express their feelings about a multitude of topics, even some that others may find distasteful or ignorant," the statement said.
A hotline has been set up in Pakistan, asking members of the public to phone in if they see offensive material anywhere.
Islamic parties say they are planning nationwide protests in Pakistan.
Five people were killed in the country in 2006 during violent demonstrations following publication of Muhammad cartoons in a Danish newspaper.
BBC website readers have been telling us what they think of the ban. Here is a selection of their comments.
I am a university student and use Facebook and Youtube as a way of interacting and staying in touch with friends. But all students are willing to give up this source of entertainment for the sake of principles.
Zahara Sohail Khan from Lahore, Pakistan
The strict policies of Facebook regarding racism and harassment are only for individual users. Now a page on Facebook is harassing billions of Muslims world-over and Facebook's management is not bothered. What hypocrisy.
Maroof from Lahore, Pakistan
I am a Muslim girl, just a normal student. When my religion is insulted, it is me who is insulted. I can live without Facebook but I definitely cannot live in humiliation. I am with my country on this and if Facebook does not take action on this, then ban or no ban, I would never go back to it anyway.
Maham Tanveer from Rawalpindi, Pakistan
As a Muslim growing up in America, I am frustrated that neither side takes the time to understand the other. For Muslims, directly insulting the sacred is beyond petty 'freedom of speech' privileges we mortals have. In the West, people think arrogantly that they are free to say anything without limits whatsoever, no matter how ridiculous or insulting.
Qureshi from Florida, USA
I am now living in Karachi, Pakistan, and I never thought I'd have to endure blocks on websites ever again after I moved from Saudi Arabia. Even though I have found a way to access blocked websites, I can't believe the government would put a ban on them.
Omar from Saudi Arabia
I did use Youtube and Facebook but I have removed my accounts from both sites and have communicated this to all my family and friends who have been using them.
Hassan Mehmood from Pakistan
This is ridiculous. I find these to be ill-advised measures. Blocking websites in countries does not prevent the content from existing in the first place. I think the Pakistani government should move to ban pornography (which is still easily available) before they ban Facebook and YouTube which are obviously on the better side of human development.
Myra from Karachi, Pakistan
This has been outrageous and infuriating. I feel disconnected from the world, from my friends, and from the easiest modes of expression available today.
Uzma from Lahore, Pakistan
It is not only Facebook and YouTube which have been blocked in Pakistan, but parts of the BBC website too, for example the link given below, many stories about Pakistan and the entire South-East Asia section. In fact, I am forced to use a proxy server just to post this comment. As a Pakistani, I feel very frustrated and angry about this crackdown on the internet and can only hope that this is temporary.