Human Rights Watch says Chinese authorities may have detained as many as "several hundred" ethnic Tibetans who travelled to India recently to receive religious instruction from the Dalai Lama, the exiled Tibetan spiritual leader.
The report quotes multiple sources as saying the detainees are being forced to attend political reeducation classes.
The report adds that the exact number of detainees is not known.
The report says it is believed to be the first time since the late 1970s that the Chinese authorities have detained a large number of ordinary Tibetans to force them to undergo political reeducation.
Authorities in China routinely force Tibetan monks and nuns to undergo patriotic education classes.
Beijing accuses the Dalai Lama of trying to separate Himalayan Tibet from the rest of China.
With agency reports