Starting 1pm on Sunday 20 May, 2012, Twitter users in Pakistan suffered a total blanket censorship  across all Internet Service Providers (ISP) in Pakistan. Some ISP have received an email from Mohammed Ali, Deputy Director (Enforcement) at Pakistan Telcommunucation Authority at around noon to block Twitter.com immediately and ordering them to flush out the cache as soon as possible.
Around 1pm Sunday afternoon internet users in Pakistan started facing the blockage, and flooded Herdict Web with reports of blockage across  all ISP:
traceroute: unknown host twitter.com
Express Tribune through its sources claimed that the block was due to a blasphemous cartoon competition: 
Express News correspondent Suhail Chaudhry had reported that the access to Twitter was blocked due to an ongoing “competition” of Prophet Muhammad’s (pbuh) caricatures. This move was made, keeping in mind the “emotions of the masses” reported Chaudhry.
Aamir Atta of ProPakistani also confirmed via his sources: 
We have got the confirmation that decision for blocking Twitter was taken yesterday on account of Draw Mohammad Day, during a meeting held at Ministry of Information Technology and chaired by Raja Parvez Ashraf, Federal Minister for Information and Technology. Meeting was attended by MoIT members, PTA and other concerned stake holders. A source who is directly aware of the meeting told ProPakistani that it was decided that Twitter will remain blocked on May 20th, 2012 to avoid any possible access of blasphemous content to Pakistani internet users.
While I opined in my personal blog that it was PTA merely testing their new url Filtering service: 
My gut feeling is that PTA is just testing their URL Filtering system, we had reports of them testing some image servers on facebook last week, and it disappeared by the evening. PTA choose Sunday to avoid any legal backlash exploiting the courts day off.
Ingrid Luden on TechCrunch wrote ‘Sure, Draw Something. Just Not The Prophet ‘:
Pictures of the Prophet Mohammad have always been a highly contentious issue — they’re not explicitly prohibited in the Qu’ran but many Sunni Muslims forbid the idea, while others do not seem to mind as much. Among the latter group are those who feel that banning such images is a restriction on freedom of expression.
Farieha Aziz comments on BoloBhi :
Speaking to Dunya TV, Minister for Information and Technology, Raja Pervaiz Ashraf said Facebook complied with the government’s request to remove blasphemous content, but Twitter refused. According to Yaseen, PTA simply passed on the orders to the ISPs and has no knowledge of how long the website is likely to remain blocked.
BytesforAll also talked about Raja Pervaiz Ashraf  ordering the block on Twitter due to blasphemous content:
Late last night, the newly appointed Minister for Information Technology, Raja Parvez Ashraf spoke to the media showing his concerns about the blasphemous content on different websites and threatened that Twitter will be banned soon if the company did not comply with the government’s orders. Within half an hour of this statement, past midnight, an official directive was issued to all Internet Service Providers in the country and Twitter started disappearing from Pakistani Cyberspace starting 10 a.m. today.
Dawn.com quotes Chairman PTA Mohammed Yaseen saying that they tried to negotatiate with Twitter  to remove the content:
We have been negotiating with them until last night, but they did not agree to remove the stuff, so we had to block it
By around 10pm, roughly nine hours after the ban started suddenly the block seemed to have been removed which resulted in tweets by Rehman Malik, Interior Minister claiming responsibility for the unblocking of Twitter:
@SenRehmanMalik : Dear All yes I spoke to PM and informed how people are feeling about it.PM ordered to reopen the twitter.I also spoke to Raja pervez abt it
Reporting the opening of the internet blockage  ProPakistani narrates:
However, as per our sources, ban was lifted before time due to bad name the incident had earned for Pakistan, especially in foreign media.
While Fiza Batool Gilani  the daughter of Prime Minister Yousuf Raza Gilani also laid claim for the Prime Minister to have restored the twitter connection.
@FizaBatoolGilan : The Prime Minister has ordered the IT Minister to restore Twitter service in Pakistan
The netizens fear that with this incident the PTA successfully tested their url filtering service which may threaten freedom of speech in the future.