The website of the Office of the Prime Minister was hacked on Wednesday, with an abusive message pasted next to an altered photograph of a laughing Prime Minister Yingluck Shinawatra.
The message cast aspersions on Ms Yingluck's intelligence and her sexual morality.
It appeared on the cabinet list page http://www.opm.go.th/opminter/mainframe.asp.
The PM's Office's website was hacked to display a message and photo (inset) by the well-known Unlimited Hack Team, who denied responsibility.
At the end of the page was a message in smaller letters saying "I know that I am the worst Prime Minister ever in Thailand history!!!" and the name "Unlimited Hack Team!!!". It has been responsible for numerous attacks, most recently on Channel 3 TV's website and the Culture Ministry's home page.
The PM's Office's site was hacked at around noon Wednesday. Technicians struggled to deal with the problem. The website was still offline at the end of the business day on Wednesday.
Information and Communication Technology Minister Anudith Nakornthap said he had been briefed about the attack on the website. The ministry is to work with police to find the hacker.
He said the ICT Ministry had dispatched a computer security team to collect digital evidence to track down the hacker.
"It is hard to estimate how long it would take the team to find the hacker because it depends on complexity and size of the computer system of the PM's Office," Gp Capt Anudith said.
Suranand Vejjajiva (Photo by Tawatchai Kemgumnerd)
"It might have been done by some teenagers... or maybe it was for political purposes,'' said Suranand Vejjajiva, the secretary-general to the PM's Office.
"Hacking a website is easy... but don't forget that checking who did it is not hard either," he told reporters.
"If we find them, they will be charged under the Computer Crime Act," he said, referring to a contentious law that rights groups say is sometimes used to muzzle online dissent.
"Unlimited Hack Team" announced on its Facebook page on Wednesday afternoon that it did not break into the website of the PM's Office.
According to the announcement, no members of the team did anything to the site. Unlimited Hack Team believed that the site was breached by another group of hackers who wanted to put blame on the team.
"Every time Unlimted Hack Team hacked a website we'll come out to claim responsibility," the announcement said. "People should wait for police to find the perpetrators and punish them."
Unlimited Hack Team has been around for a while. According to a lengthy video posted on YouTube, the "team" comprises two young men, who use well-known exploits and scripts to break into websites from a regular Windows Server PC.
The team posted a list of instructions, dated last New Year's Eve, on "how to avoid being hacked by the Unlimited Hack Team" but it appeared it was not taken seriously by administrators of sites including the Culture Ministry, TV Channel 3 and, now, the PM's Office.
The incident comes days after Ms Yingluck filed a defamation case was filed against Chai Rachawat, a cartoonist for the Thai Rath newspaper, for allegedly comparing her with a prostitute on his Facebook page.
His post apparently took aim at the premier's appearance at a democracy forum a day earlier in Mongolia, where she gave an unusually fiery speech condemning the 2006 coup that toppled her brother Thaksin as premier.
Since then debate has flared over what is an acceptable level of criticism of Ms Yingluck.
On Sunday, ICT Minister Anudith told reporters that websites that allow criticism of Prime Minister Yingluck Shinawatra will be shut down. His comment led to criticism from the opposition saying such action is a violation of democratic principles.
On Tuesday, Gp Capt Anudith backed down and said any such decision lies with the Criminal Court.