Pakistan blocked Twitter on Sunday, saying the website had refused to remove posts promoting a Facebook competition involving caricatures of the Prophet Mohammed.
"The website has been banned by the Ministry of Information Technology and the decision was conveyed to us," said Mohammad Younis Khan, spokesman for Pakistan Telecommunication Authority (PTA).
"Both Facebook and Twitter were involved. We negotiated with both. Facebook has agreed to remove the stuff but Twitter is not responding to us."
Twitter had been blocked but Facebook was still available, he said, adding that those responsible for the competition were "trying to hurt Muslim feelings".
Responding to the furore around the ban, one Twitter user, @vinodvyas, wrote: "Now billions of ppl know there exists a competition to draw Prophet."
Twitter is widely used in Pakistan, including by prominent public figures such as celebrities, cricketers, cabinet ministers and members of parliament.
Former president Pervez Musharraf, in exile in Britain, regularly tweets, as does Interior Minister Rehman Malik, and Ali Zafar, the popular actor and musician. Asma Jahangir, the leading lawyer, is also on Twitter.
The Ministry of IT on Sunday also directed the telecommunication authority to remain alert and block immediately all links displaying what it deemed profane caricatures of religious figures.
Muslims across the globe staged angry protests over the publication of satirical cartoons of Mohammed in European newspapers four years ago.
A suicide attack outside the Danish embassy in Islamabad that year killed eight people. Al-Qaeda claimed the attack to avenge the cartoons.
A court in Pakistan blocked Facebook in May 2010 because of a similar competition organised by an anonymous Facebook user who called on people to draw the Prophet to promote "freedom of expression".
The competition sparked a major backlash in conservative Muslim Pakistan, where even moderates were deeply offended by the drawings that appeared on the "Everyone Draw Mohammed Day" Facebook page.
The competition saw Facebook blocked for almost two weeks after a petition by a group of Islamic lawyers. The PTA also banned YouTube for a week and restricted access to other websites, including Wikipedia, lashing out against "growing sacrilegious" content.
The government at the time said it would conduct monitoring of major websites for anti-Islamic content.
Ali Dayan Hasan, Pakistan director at Human Rights Watch, said the latest ban was "ill-advised, counter-productive and will ultimately prove to be futile as all such attempts at censorship have proved to be".
"The right to free speech is non-negotiable and if Pakistan is the rights-respecting democracy it claims to be, this ban must be lifted forthwith," he said.
