GENEVA — China is systematically undermining international human rights groups in a bid to silence critics of its crackdown on such rights at home, a watchdog organization said on Tuesday. The group also faulted the United Nations for failing to prevent the effort, and at times being complicit in it.

“China’s crackdown on human rights activists is the most severe since the Tiananmen Square democracy movement 25 years ago,” Kenneth Roth, the director of the agency, Human Rights Watch, said in Geneva on Tuesday at the introduction of a report that he described as an international “wake-up call.” “What’s less appreciated is the lengths to which China goes to prevent criticism of that record of oppression by people outside China, particularly those at the United Nations.”

“The stakes are not simply human rights for the one-sixth of the world’s population who live in China,” Mr. Roth added, “but also the survival and effectiveness of the U.N. human rights system for everyone around the globe.”

The report highlights China’s measures to prevent activists from leaving the country to attend meetings at the United Nations, its harassment of those who do manage to attend and the risk of reprisals when they return or if they interact with United Nations investigators inside or outside China.