Human rights advocates say they fear that with the Arab Spring lending new regional influence to Turkey, the United States and Europe are turning a blind eye to encroaching authoritarianism there. “Turkey’s democracy may be a good benchmark when compared with Egypt, Libya or Syria,” said Hakan Altinay, a senior fellow at the Brookings Institution. “But the whole region will suffer if Turkey is allowed to disregard the values of liberal democracy.”

Among the most glaring breaches of press freedom, human rights advocates say, was the arrest of Mr. Sener, 45, a German-born reporter who was working for the newspaper Milliyet at the time of his arrest. In 2010 he won the International Press Institute’s World Press Freedom Hero award for his reporting on the murder of Hrant Dink, a prominent Turkish-Armenian journalist who was assassinated in Istanbul in 2007.

Mr. Sener said he believed that he was in jail because he dared to write a book criticizing the Turkish state’s negligence in failing to prevent Mr. Dink’s murder. His defense team says the prosecution’s case rests on spurious evidence, including a file bearing his name that an independent team of computer engineers concluded had been mysteriously installed by a virus on a computer belonging to OdaTV, an antigovernment Web site. He was held for seven months without charges. If convicted, he faces up to 15 years in jail.

“Nedim Sener is being accused on the basis of rumors and fantasies,” said his lawyer, Yucel Dosemeci. “He is being targeted to create a culture of fear.”

In late December, Turkey drew fresh criticism after the police detained at least 38 people, many of them journalists, saying they had possible links to a Kurdish separatist group. But critics say dozens have been arrested whose only offense was to have expressed general support for the rights of Kurds, a long-oppressed minority here.

Over the past year, the government has been arresting prominent critics like Mr. Sener, as well as dozens of current and former military personnel, intellectuals and politicians who have been linked to what officials say was a plot to overthrow the government by an organization called Ergenekon.

Four years into the investigation, no one among the more than 300 suspects charged in the case has been convicted, even though courts have heard more than 8,000 pages worth of indictments, many of them based on transcripts of surreptitiously recorded private telephone conversations.