Hearings have begun in a Thai court in a criminal libel case filed against an activist and a local newspaper by a corporation that was founded by Thailand's prime minister. Some media groups are concerned that litigation is being used in an attempt to restrict Thailand's open and lively press.

The private criminal prosecution that opened Tuesday pits the Shin Corporation, a major Thai communications firm, against the Thai Post newspaper and Supinya Klangnarong, head of the Campaign for Popular Media Reform.

The firm has separately filed a $10 million civil lawsuit against Ms. Supinya and officials of the newspaper.

Shin Corp., which is pressing the case, says its reputation was unfairly damaged by a July 2003 article in which Ms. Supinya was quoted as saying the company's profits had tripled because of its political connections. The article said government policies had direct benefits for corporate allies of the prime minister.

The company accuses Ms. Supinya of making irresponsible and false comments, and says its higher profits were the result of good management and not political connections.

Shin Corp is a telecommunications, satellite and media company that was founded by Thaksin Shinawatra before he became prime minister in 2001. It is now controlled by members of his family.

Shawn Crispin, Asia Program consultant with the U.S.-based Committee to Protect Journalists, says the case raises concerns over the use of litigation to curb the press.

"Freedom of expression advocates are concerned about the use of criminal defamation, and the follow-up civil cases have a real chilling effect on freedom of the press in countries that have criminal defamation laws on the books," he said.

Ms. Supinya says she is confident she will win. Outside the court Tuesday, she said the case is a test of freedom of the press in Thailand.

"It's going to be a standard - local or international - and be mentioned about this for a long while so that [is] why the result is quite important, not only for me but for the freedom of the press and freedom of expression in Thailand as well," she said.

The trial is expected to last four months, and Ms. Supinya's defense team plans to call up to 40 witnesses. She has received significant support from local and international journalistic organizations.

There have been other recent concerns about freedom of expression in Thailand. In June, the Information and Communication Technology Ministry closed down two Web sites that had been critical of the government.

Kallaya Soponpanich, a member of the opposition Democrat Party, was at the courthouse Tuesday to support Ms. Supinya. Mrs. Kallaya believes the case may affect public support for Mr. Thaksin's government.

"The situation is rather sensitive - it could be either way. It's very, very difficult to foresee how serious, but the public awareness and support [for her] more and more," she said.

Thailand has long had the reputation of having some of the freest media in Southeast Asia. But Mr. Crispin of the Committee to Protect Journalists says that reputation is being harmed by this case, and by the recent allegations of interference in both print and broadcast media.