BEIJING — At a time when China’s government has granted the foreign press greater freedom, it is tightening controls on Chinese who write about politically sensitive or embarrassing topics, human-rights activists and journalists say.

International PEN, a writer’s organization that calls itself the world’s oldest human-rights group, said yesterday that local police prevented 20 Chinese writers from attending its international conference in Hong Kong over the weekend.

Some were warned not to go, while others who had permits to travel to Hong Kong, a Chinese territory under separate administration, had their documents seized at the border.

The travel restrictions came after China’s recent ban of eight books, most of them works of history, including one about the outbreak of severe acute respiratory syndrome, or SARS, in 2003. The Chinese government was criticized for being too slow in confronting the deadly new virus.

The crackdown came just weeks after the government relaxed decades-old restrictions on foreign journalists, giving them greater freedom to report the 2008 Beijing Olympics — a move that it hoped would burnish its international image.

But Chinese writers said the tolerance granted foreigners does not extend to those who write for a Chinese audience.

“It’s all for show,” a writer who has been blacklisted and unable to publish under his own name for more than two years, Yu Jie, said. “They’re actually tightening their grip on China’s writers.”

A prolific Internet essayist and political critic, Liu Xiaobo, said authorities rejected his application for a permit to attend the International PEN event in Hong Kong.

“They have different regulations for the outside and for us on the inside,” Mr. Liu said. “The new openness that they talk about is all about the Olympics. Nothing has changed for the people within the country. … It’s barbaric.”

Fifteen Chinese writers did attend but the 20 who didn’t either could not obtain travel documents or were told not to go, general secretary of the Independent Chinese PEN Center, Zhang Yu, said.

Zhang Yihe, whose book on the repression of classical Chinese opera stars in the 1960s was among eight recently banned, decided not to attend the conference after being warned by authorities. A former journalist, Zan Aizong, and another dissident writer, Zhao Dagong, had travel papers but were blocked from leaving, PEN said.

While the Chinese leadership has sought to create the appearance of a more open society in advance of the Olympics, critics say it has actually grown less tolerant of dissent under President Hu — whose government has imposed new restrictions on religion, the press, political activism, and the Internet.

“China did itself a p.r. favor” by loosening restrictions on foreign journalists, Kristin Jones of the Committee to Protect Journalists said. The move, however, “doesn’t address the more serious risks faced by Chinese journalists,” she said.

Ms. Jones, who is a senior research associate for Asia, said not only do domestic journalists face greater restrictions on their work but they are also more likely to be threatened or physically attacked by local authorities than foreigners are.

Authorities continue to use vaguely worded state secrecy and subversion charges to suppress criticism of the ruling Communist Party.

“It’s a double standard because the Chinese government knows that if these people are allowed to travel, to move, to speak freely, they will reach new audiences,” a spokesman for the Paris-based advocacy group Reporters Without Borders, Vincent Brossel, said.

“They know the impact will increase if these people … especially human-rights defenders, become an icon for the fight for democracy,” he said. “It will be much more difficult for authorities to crack down on them.”

The crackdown has hit China’s whistleblowers as well as its journalists.

An elderly doctor who embarrassed Chinese leaders by exposing blood-selling schemes that infected thousands with HIV was detained by authorities at her home, apparently to prevent her from applying for an American visa, fellow AIDS activists said yesterday.

Gao Yaojie — who is in her 80s — was to be honored next month in Washington by Vital Voices Global Partnership, an international group that provides aid and training to women who serve as community leaders. But Ms. Gao was warned last week not go, a fellow campaigner and friend, Hu Jia, said, adding that police detained Ms. Gao at her home in central Henan province before she could leave for a planned trip to Beijing on Sunday to arrange her visa.

It was at least the second time authorities have tried to stop her from traveling abroad. In 2001, Ms. Gao was refused a passport to go to Washington to accept an award from a U.N. group.

She gained recognition in the late 1990s for her efforts to alert people in Henan to an AIDS outbreak being spread by tainted blood transfusions. Her warning came at a time when the government was still tight-lipped about its problem with the disease.

Ms. Gao spoke openly to the press and distributed brochures about the spread of AIDS among poor farmers through commercial blood banks. She has distributed medicine, cared for AIDS orphans, and hosted AIDS sufferers in her modest apartment.