The Office of the Prime Minister's website was hacked Wednesday, with messages and a photograph deriding Yingluck Shinawatra posted on its homepage.
One message appeared alongside a doctored image of Ms Yingluck laughing. It made derogatory remarks about the premier's intelligence and sexual morality.
At the bottom of the page was another message in smaller letters saying: "I know that I am the worst prime minister ever in Thailand's history!!!", along with the name "Unlimited Hack Team!!!". The group, however, posted a message on its Facebook page denying responsibility for the attack.
The hacking "team" had claimed responsibility for hacking into Channel 3 TV's website after it banned the political drama Nua Mek 2.
- Earlier report: PM's website hacked
The latest attack appeared to be a direct response to Ms Yingluck's call on Tuesday for the Information and Communications Technology (ICT) Ministry to take action against people spreading groundless accusations against her online.
She made the call in response to growing online criticism after her decision to file a defamation suit against newspaper cartoonist Chai Ratchawat, who called her an "evil woman" on his personal Facebook page.
The PM's Office's site was hacked around noon. It was taken down for several hours soon after.
ICT Minister Anudith Nakornthap said his ministry was working with police to track down those responsible. He said he did not believe the incident was related to the ICT Ministry's clampdown on people posting insulting messages against Ms Yingluck. The ministry will conclude its probe into the hacking Thursday.
PM's Office secretary-general Suranand Vejjajiva said gains in technology had made it easier to hack a website, but also for authorities to track down those responsible.
The hacker or hackers, if found, will be prosecuted according to the law, he said.
In July last year, Ms Yingluck's Twitter account was hacked and eight messages criticising her policies were posted.
One of the tweets said: "What opportunities do the poor really have? We are deceiving the poor. We gave them hope to gain their votes so that our [cronies] can benefit." A day later, officials arrested a Chulalongkorn University architecture student who confessed to using Ms Yingluck's password to gain access to her account.
Prinya Hom-anake, a computer security expert, said hacking a website is not complicated. Up to 2,960 websites were hacked from September 2012 to January this year, 1,250 of which were government ones.
Around noon on Wednesday, the inline list of cabinet ministers on the web page of the PM's Office at pmo.go.th was replaced with a crude attack on Prime Minister Yingluck Shinawatra.
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