The global head of Human Rights Watch said he was denied entry on Sunday to Hong Kong, where he was scheduled to launch the organization's latest world report this week.
Kenneth Roth, the group's executive director, said he was blocked at Hong Kong airport from entering for the first time, having entered freely in the past.
During seven months of sometimes violent anti-government protests, the Chinese-ruled city has barred several activists, foreign journalists and an academic.
"This year (the new world report) describes how the Chinese government is undermining the international human rights system. But the authorities just blocked my entrance to Hong Kong, illustrating the worsening problem," Roth said in a post on his Twitter account.
He added that Hong Kong immigration officials had cited only "immigration reasons".
Speaking at a daily news briefing, Chinese Foreign Ministry spokesman Geng Shuang said Human Rights Watch had encouraged radicals in Hong Kong "to take violent and extremist actions."
"They have instigated the activities of Hong Kong separatists and hold a major responsibility for the current chaos," Geng said.
Responding to Reuters' questions, a spokesman for the Hong Kong Immigration Department said it did not comment on individual cases.
"In handling each immigration case, (the department) will, in accordance with the laws and immigration policies, fully consider all relevant factors and circumstances of the case before deciding whether the entry should be allowed or not," the spokesman said.
Human Rights Watch, based in New York, had been scheduled to release its 652-page World Report 2020 at the Foreign Correspondent Club in Hong Kong on Jan. 15.
Roth would now launch the report on Jan. 14 at the United Nations in New York, the organization said in a statement.
"This disappointing action is yet another sign that Beijing is tightening its oppressive grip on Hong Kong and further restricting the limited freedom Hong Kong people enjoy under 'one country, two systems'," Roth said in the organization's statement.
Human Rights Watch said a Chinese Ministry of Foreign Affairs official had threatened to impose unspecified "sanctions" against it and several US-based pro-democracy organizations in early December. Neither Beijing nor Hong Kong authorities have since provided further details, it added.
The Hong Kong Foreign Correspondents' Club, said in a statement it is concerned that the Hong Kong government is using the immigration department to act punitively against organizations and media representatives it does not agree with.
"This sort of treatment and lack of explanation appeared to be making a weapon of visas and violated press freedom rights in Hong Kong law," the Club said.
Many people in Hong Kong are angered by what they see as Beijing's ever-tightening grip on the city, which was promised a high degree of autonomy under a "one country, two systems" framework when the former British colony returned to Chinese rule in 1997.
Beijing denies interference and blames the West for fomenting the unrest.