Pakistan had blocked the micro-blogging website Twitter because it refused to remove material considered offensive to Islam, said one of the country's top telecommunications officials.
The material was promoting a competition on Facebook to post images of the Prophet Muhammad, Mohammad Yaseen, chairman of the Pakistan Telecommunication's Authority, said on Sunday.
Many Muslims regard depictions of the prophet, even favourable ones, as blasphemous.
Pakistani newspaper websites said there was no indication as to how long the site would be down.
Yaseen said Facebook had agreed to address Pakistan's concerns but officials had not been able to get Twitter to do the same, the Dawn and Express Tribune newspapers reported.
Members of the Pakistani government were in negotiations with Twitter officials over the issue, the Express Tribune said.
"We have been negotiating with them [Twitter] until last night, but they did not agree to remove the stuff, so we had to block it,'' said Yaseen.
"The ministry officials are still trying to make them agree, and once they remove that stuff, the site will be unblocked."
Instructions to temporarily block Twitter came from Pakistan's ministry of information technology, said Yaseen.
Al Jazeera's reporters in Pakistan confirmed that they were unable to access the Twitter website earlier on Sunday.
Al Jazeera's Asad Hashim in Karachi said he could not access the site from his mobile phone, but was still able to see tweets posted earlier in the day. He said it appeared that the ban had therefore come into effect sometime about 8:00GMT (1pm local time).
By 17GMT, Hashim said the ban had been lifted.
'Cheap moral stunt'
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.
There was widespread criticism of the government's action by those on the microblogging site, who tend to be more liberal than average Pakistanis.
"Another cheap moral stunt by Pakistan," tweeted liberal Pakistani columnist Nadeem Paracha.
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.
Shahzad Ahmad, an Islmaabad-based activist with Bytes For All, a rights organisation focusing on information technology, told Al Jazeera that the ban was bound to be short-lived.
"They cannot ban any communication channel for that long," said Ahmad, who added that the Pakistani government has a history of blocking Youtube, Facebook, Flikr and Wikipedia.
"This is uncalled for, and we really don't know why they've blocked yet another social networking Website...frankly, the way we see it, this is bringing embarrassment. It is bringing egg on the face of the government," said Ahmad.
A top court in Pakistan ordered a ban on Facebook in 2010 amid anger over a similar competition. The ban was lifted about two weeks later, after Facebook blocked the particular page in Pakistan.
That controversy sparked many in the country's liberal elite to question why Pakistanis could not be entrusted to decide for themselves whether or not to look at a website.
Some observers noted that Pakistan had gone further than several other Muslim countries by banning Facebook, and
said it showed the rise of conservative Islam in the country.
There were a handful of protests against Facebook back in 2010, often organized by student members of radical Islamic groups. Some of the protesters carried signs advocating holy war against the website for allowing the competition page to be posted in the first place.
Follow the issue on Twitter by searching the hashtag #TwitterBan