Pakistan has blocked Twitter over "blasphemous content" on the microblogging website, according to a senior government official.
The site had reportedly refused to remove tweets promoting a Facebook competition involving caricatures of the Prophet Mohammed.
Islam strictly prohibits the depiction of any prophet as blasphemous.
"This has been done under the directions of the ministry of information technology. It's because of blasphemous content," Mohammed Yaseen, chairman of the Pakistan Telecommunication Authority (PTA), told Reuters.
"They (the ministry) have been discussing with them (Twitter) for some time now requesting them to remove some particular content."
He added: "We have been negotiating with them until Saturday night, but they did not agree to remove the stuff, so we had to block it."
Mr Yaseen said Facebook had agreed to address Pakistan's concerns about the competition, but officials failed to get Twitter to do the same.
He did not specify which users or messages had prompted the ban.
Asked about the nature of the content, Mohammad Younis Khan, spokesman for the PTA, told AFP there had been a planned competition to "post caricatures of Prophet Mohammed".
The Internet Service Providers Association of Pakistan said its members had been asked to block Twitter indefinitely, but had not been given a reason by the government.
It is not the first time the country has banned the use of social networking sites because of content considered offensive to Islam.
Access to Facebook, Twitter, YouTube and other websites was blocked for nearly two weeks in May 2010 over blasphemous material.