Human Rights Watch, citing "multiple" unidentified sources, said several hundred Tibetans "may have been" arrested by local police in the Tibetan Autonomous Region and forced to undergo political re-education.
Sophie Richardson, director of the China program at Human Rights Watch, said from New York that such actions are in violation of international law.
"Authorities in the region should release these individuals, as their detention only escalates the tension in Tibetan regions, which already have increased limits on travel and communication as well as troop and security presence," she said in a statement.
Tibetans frustrated with Chinese oppression have committed suicide as an act of protest. Chinese authorities said some Tibetans committed acts of violence.
Human Rights Watch said the crackdown was the first time Chinese authorities have taken such action against such a large number of Tibetans since the 1970s. The group added that Beijing let around 7,000 Tibetans into Nepal or India recently but have since taken a harder line in light of the recent Tibetan uprising.