Pakistan on Sunday blocked Twitter over a competition to make caricatures of the Prophet Mohammed on the social networking site, officials told AFP.
"The website has been banned by Ministry of Information Technology and the decision was conveyed to us. There was blasphemous material on Twitter," said Mohammad Younis Khan, spokesman for Pakistan Telecommunication Authority (PTA).
Asked about the nature of the "blasphemous" content, Khan said there had been a planned competition to "post caricatures of Prophet Mohammed".
An official from the ministry said it had requested several times that no such competition be put on Twitter but the website had failed to respond.
Islam strictly prohibits the depiction of any prophet as blasphemous. Muslims across the globe staged angry protests over the publication of satirical cartoons of Mohammed in European newspapers four years ago.
In 2008, a suicide attack outside the Danish embassy in Islamabad killed eight people. Al-Qaeda claimed the attack to avenge the cartoons.
Pakistan blocked Facebook and YouTube in mid-2010 because of similar "blasphemous" content.