Rights groups have expressed their fears that a media baron's attempts to buy into two of Thailand's major newspapers will erode press freedom.
GMM Grammy is trying to take over the publishers of Matichon
Paiboon Damrongchaitham, head of the huge GMM Grammy group, announced plans to buy a stake in Matichon and the Bangkok Post earlier this week.
Mr Paiboon is close to Prime Minister Thaksin Shinawatra, but has denied any political motivation in his decision.
Hundreds of Thai journalists reportedly wore black on Thursday in protest.
Mr Thaksin is widely accused of trying to curb the Thai press, and companies associated with his family have already acquired several television and radio stations, including iTV, Thailand's only private television station.
Both the newspapers Mr Paiboon intends to buy into - the Thai-language Matichon and Post Publishing, owner of the English language Bangkok Post - currently face libel suits from government agencies or companies linked to ministers.
Mr Paiboon has denied buying into the newspapers to aid Mr Thaksin.
"I will not get involved [in the newspapers' operations] or change any editorial policies," Mr Paiboon told a press conference on Tuesday.
"I will not send any of my men to be board members, even if that's hard to believe," he added.
But his critics remain unconvinced.
"Independence is being and will be compromised," said Roby Alampay, who heads the Bangkok-based South East Asian Press Alliance.
"When you look at the context... the intertwined interests and relationships between business and politics and government in Thailand has long been leading up to developments like this," he said on Thursday.
Darunee Hiranrak, president of the Thai Chamber of Commerce University, told the French news agency AFP that although the takeover was legal, the company still needed to answer questions about whether it was ethical and transparent.
GMM Grammy is Thailand's largest entertainment company, owning 12 radio stations as well as television and radio production firms and several magazines.
Mr Paiboon made headlines last year when he helped Mr Thaksin in his attempt to buy the English premiership football club Liverpool.
The deal came to nothing, but highlighted the close links between the two men.