Pakistani protesters shouted "Death to Facebook", "Death to America" and burnt US flags on Friday, venting growing anger over "sacrilegious" caricatures of the Prophet Mohammed on the Internet.
A Facebook user organised an "Everyone Draw Mohammed Day" competition to promote "freedom of expression", inspired by an American woman cartoonist, but sparked a major backlash in the conservative Muslim country of 170 million.
Islam strictly prohibits the depiction of any prophet as blasphemous and the row has sparked comparison with protests across the Muslim world over the publication of satirical cartoons of Mohammed in European newspapers in 2006.
The Pakistan Telecommunications Authority (PTA) banned access to Facebook, YouTube and more than 450 links, including restricted access to Wikipedia in view of what it called "growing sacrilegious content".
PTA released a toll-free telephone number and email address, and has acted on complaints received by the regulator.
Up to 3,000 people rallied in the eastern city of Lahore at the behest of a coalition of Islamic groups, including Jamaat-ud Dawa, regarded as a front for the militant group blamed for the 2008 Mumbai attacks.
"This is a war and we have to show unity," Farid Ahmed Paracha, a central leader of hardline Sunni Muslim political party Jamaat-e-Islami told the crowd.
"We should tell America that this the final match," he added.
Shouting anti America and anti Facebook slogans with chanting "Death to America," the participants burnt US, Norway, Sweden and Denmark flags.
In Karachi, Pakistan's biggest city, religious parties mobilised hundreds of protesters on to the streets shouting "Death to Facebook", "Death to America" and branded the United States the "root cause of all mischief."
In Multan, a shrine city in Punjab province, hundreds of people rallied, burning US flags and tyres to block traffic before dispersing peacefully.
About 250 people demonstrated in the garrison city of Rawalpindi, as well as in the northwestern city of Peshawar, where they chanted "Death to Facebook, death to Youtube," an AFP reporter said.
But despite general anger over the caricatures, the ban on websites has sparked some criticism, particularly among the largely Western-educated elite living and working in the relatively moderate Lahore and Karachi.
The offending Facebook page has attracted 105,000 fans -- and five pages of crude manipulated pictures and caricatures. Pages denouncing the competition and calling for a boycott of the May 20 competition attracted far more fans.
Facebook expressed disappointment at being blocked and said it was considering whether to make the offending page inaccessible in Pakistan.
YouTube, the Google-owned video-sharing site, said it was "working to ensure that the service is restored as soon as possible".
The controversy has yet to incite a mass outpouring on to the streets in Pakistan, where there are an estimated 2.5 million Facebook users, and it remains to be seen how far protests will spread to other Muslim countries.
Sweden said it has closed its embassy in Islamabad for more than two weeks due to the security situation, refusing to say whether any direct threats had been issued against the mission.
An Al-Qaeda front organisation has offered 100,000 dollars to anyone who kills Swedish artist Lars Vilks, who has angered many Muslims by drawing highly blasphemous caricature of the Prophet.
Pakistan condemned the caricatures on Facebook and said that "such malicious and insulting attacks hurt the feelings of Muslims around the world".
The PTA asked Facebook and YouTube, which are wildly popular in Pakistan and set up in the United States, to resolve the matter as soon as possible in a manner that "ensures religious harmony and respect."
The purported creator of the Facebook page told a US television channel in a voice-only interview that he had meant to stand up for "freedom of expression."
A rival Facebook page called "Against Everybody Draw Mohammed Day," which was started to oppose the caricature page, had drawn some 106,300 fans.
Molly Norris, the American cartoonist whose work inspired the controversial page, condemned the Facebook spin-off and apologised to Muslims.
She drew a cartoon in April to protest against the cancellation of an episode of popular show "South Park." Norris satirically proposed May 20 as an "Everybody Draw Mohammed Day."