BEIJING, Tuesday, Dec. 14 - The Chinese police on Monday afternoon detained three leading intellectuals who have been critical of the government, apparently stepping up a campaign to silence public dissent.

Yu Jie and Liu Xiaobo, literary figures, and Zhang Zuhua, a political theorist, were detained in raids at their homes, relatives and friends said. Mr. Yu's relatives were handed a warrant that said he was suspected of "participating in activities harmful to the state," said his wife, Liu Min.

Friends said Mr. Yu and Mr. Zhang were released Tuesday morning after police confiscated materials and searched their computers. The status of Mr. Liu remains uncertain, and it is not clear whether any of the three will face charges.

The detentions were the latest in a string of arrests and official harassment of journalists, writers and scholars who have spoken out against government policies or written articles or essays that officials have deemed damaging.

Since President Hu Jintao replaced Jiang Zemin as China's military chief in September, leaving Mr. Hu in full command of China's government, ruling party and army, analysts say the political environment has become more repressive. The scope for discussing sensitive topics in the state-run media has decreased, they said, while the authorities appear intent on punishing people who violate unwritten rules about the limits on free speech.

The crackdown could signal an effort by Mr. Hu to dispel hopes - already greatly diminished - that he would usher in a period of relative political relaxation when he consolidated power. Instead, there are signs that he is seeking to manage state affairs in a more hands-on and less permissive style than that associated with Mr. Jiang, who in his later years focused on carrying out broad economic changes while allowing a degree of media openness.

Political analysts said Mr. Hu had made clear that he intended to restore discipline to China's increasingly diverse news media while at least temporarily restricting space for leading intellectuals to voice freely their views about politics, economics and media management.

Editors say the Propaganda Department has received a fresh mandate to micromanage daily news coverage and to ban coverage of an increasingly long list of sensitive issues, including broad topics like China's growing social inequality.

Mr. Yu and Mr. Liu have been outspoken critics for many years. Mr. Liu was accused of serving as an organizer of the Chinese democracy movement in 1989, which ended in the violent crackdown on dissent in June of that year. He spent several years in prison in the 1990's.

Three years ago, the two founded a Chinese chapter of the PEN organization, which defended writers, poets and journalists persecuted by the government. The men wrote a series of articles this fall supporting Shi Tao, a poet and journalist based in Hunan. Mr. Shi was arrested recently and accused of leaking state secrets, apparently while working as a reporter in Shanghai.

PEN, which operates without official support, convened an award ceremony in late October to honor Zhang Yihe, the author of a memoir about the repressive "antirightist" political campaign in the late 1950's. The book was banned.

Zhang Zuhua, the political theorist, is an associate of Mr. Yu and Mr. Liu who attended the award ceremony, friends said.

Liu Min, Mr. Yu's wife, said in a brief telephone conversation on Monday evening that her apartment building was surrounded by uniformed policemen. The telephone line to her home was cut in the middle of a conversation with a reporter.

The detentions appear to be part of a pattern.

Jiao Guobiao, a journalism professor at Beijing University who wrote a scathing criticism of the Propaganda Department earlier this year, was stripped of his teaching responsibilities recently. Wang Guangze, an editor at the state-run 21st Century Business Herald, was fired after returning from a speaking engagement in the United States last month. There, he had discussed how the Internet was affecting China's politics.

In September, the authorities arrested Zhao Yan, a local journalist who worked for the New York Times bureau in Beijing, on charges of providing state secrets to foreigners. Some of Mr. Zhao's friends say they believe that State Security officials are seeking to tie Mr. Zhao to an article published in The Times that reported an offer by Mr. Jiang to retire two weeks before the leadership change was announced.

"The steps the authorities have taken are not by coincidence," said an editor at a major state-run newspaper who asked not to be identified. "This is a new era, but right now we're moving backward."

Christopher Buckley contributed reporting for this article.