BEIJING — A prominent Tibetan businessman and philanthropist who was lauded by Chinese officials for his environmental work has been sentenced to 15 years in prison on charges that he traded in looted relics more than a decade ago, his lawyer said Thursday night.

The trial of the Tibetan, Karma Samdrup, 42, was widely viewed as an effort to punish him for his outspoken defense of his two brothers, both of whom had publicly berated a local police chief who hunted endangered species in a Tibetan nature preserve. Mr. Samdrup maintained his innocence and said he had been tortured during six months of custody in an effort to force a written confession to charges of “grave robbing.”

Mr. Samdrup plans to appeal the conviction, the lawyer said.

Human rights advocates say the prosecution of Mr. Samdrup and his brothers are part of a worrisome crackdown on Tibetan artists, intellectuals and students, among others, that has intensified since March 2008, when rioting in Tibet stunned Chinese leaders in the midst of their preparations for the Olympics in Beijing.

Although Mr. Samdrup is the most high-profile Tibetan figure to be jailed in the last two years, exile groups say the authorities have detained at least 50 people, many of them educated members of the Tibetan middle class who had previously stayed away from the region’s politics.

“This conviction is going to set back China’s policies in Tibetan areas and further depress the hope and confidence people had in the Chinese government,” said Robbie Barnett, director of the modern Tibetan studies program at Columbia University.

Mr. Samdrup’s lawyer, Pu Zhiqiang, complained that the trial was plagued by irregularities, including altered evidence and the sudden appearance on Wednesday of a mysterious witness. He also said the judge refused to delve into Mr. Samdrup’s claims that he was beaten and drugged during his six months in custody.

“The court completely ignored the facts, trampled on the legal system and violated Karma’s humanity,” Mr. Pu said.

The sentence also included a $1,500 fine and the deprivation of Mr. Samdrup’s political rights for five years after his release.

Named “philanthropist of the year” in 2006 by the state broadcaster CCTV, Mr. Samdrup was once embraced by the Chinese Communist Party for his environmental work and his willingness to give the government pieces from his art collection.

But his fortunes turned last year after his brothers, both environmentalists, were detained for their public campaign against a police official in Chamdo Prefecture who had a fondness for hunting protected animals. One brother was sentenced to 21 months in a labor camp for “harming national security.” The other is still awaiting trial.

After visiting the men in detention and hearing their tales of mistreatment, Mr. Samdrup began agitating for their release. Last January, he, too, was arrested and accused of buying a carpet, clothing and a wooden coffin that had been pilfered from an ancient tomb in the far western region of Xinjiang. The accusations, however, stemmed from a 1998 charge that the police had declined to pursue at the time.

Even with a high-profile lawyer from Beijing, Mr. Samdrup did not stand much of a chance, his wife, Zhenga Cuomao, said in a telephone interview from Xinjiang, where the hearing took place. “I felt like this was not a real trial, but that they just went through the motions to reach a predetermined verdict,” she said.

Ms. Cuomao, who was allowed into court but not permitted to speak to her husband, said he told the judge about enduring days without food or sleep and said his interrogators soaked him in cold water in the dead of winter. “He never signed the confession because he knows he is innocent,” she said.

She said she had yet to tell the couple’s two daughters, 8 and 10, that their father was in jail. Since his arrest, she had said he was studying abroad. “I won’t tell them the truth anytime soon,” she said. “I can’t bear to break their hearts.”

Nicholas Bequelin, a researcher at Human Rights Watch, agreed with Ms. Cuomao’s evaluation of the trial, noting that the 10-page verdict released Thursday night came several hours after the end of the trial, strongly suggesting that the decision was preordained.

“This conviction is not only a huge blow for the Chinese legal system but for Tibetan-Chinese relations,” Mr. Bequelin said.