The Democrat Party slammed Information and Communication Technology (ICT) Minister Anudith Nakornthap on Monday for his threat to silence websites that allow criticism of the prime minister.
Opposition leader Abhisit Vejjajiva said he was surprised by Gp Capt Anudith's threat.
It was a violation of democratic principles and at odds with Prime Minister Yingluck Shinawatra's promise in her controversial speech in Mongolia to promote democracy in Thailand.
In a democracy, the government is open to criticism, the Democrat Party leader said.
The ICT minister declared on Sunday that his ministry would take prompt action against the owners of websites allowing people to post messages criticising the prime minister, threatening fines and jail terms, and warning they could be shut down.
He said they could be fined up to 20,000 baht or put in prison for one year under the provisions of the Criminal Code.
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The minister's rant came in the wake of a post by Somchai Katanyatanan, a popular cartoonist for the mass circulation Thai Rath newspaper under the name of Chai Ratchawat. He implied the prime minister was an ''evil woman'' in a comment on her speech made at a democracy forum in Ulan Bator, Mongolia, on April 29.
The cartoonist wrote on April 30 on his Facebook page, "Prostitutes are not evil. They just sell their bodies. But an evil woman sells the nation." His message was widely circulated in social media, sparking debates in cyberspace.
Ms Yingluck, in her speech, blasted the 2006 coup which ousted her brother, Thaksin, warned of continued attempts by a group of people trying to ''derail'' democracy and claimed snipers were shooting red-shirts during the 2010 protests in Bangkok.
Critics were upset by her remarks, accusing her of telling half-truths, of defending her fugitive brother and intentionally omitting any mention of the widespread corruption and interference in independent agencies by the Thaksin administration, which were among reasons cited for the coup in 2006.
The prime minister sued the cartoonist for defamation on Friday.
Democrat deputy spokeswoman Malika Boonmeetrakul said Gp Capt Anudith went beyond his authority in his attempt to fend off criticism of the prime minister. The minister had no power to close websites, which could be shut down only by a court order, she said.
Users of social media, along with the press, have the right to freedom of expression and to comment on and criticise public figures, including the prime minister. Ms Yingluck could take legal action against anyone she believed defamed her, Mrs Malika said.
Meanwhile, a police inquiry will determine whether Mr Somchai broke the law in posting his comment.
Pol Maj Gen Anuchai Lekbumrung, deputy chief of the Metropolitan Police Bureau, said on Monday that he expected the panel to report back on Tuesday.
He said Pol Maj Gen Wichanwat Borirakkun, the city police Division 1 commander, would appoint the inquiry members.