More than 1,200 Tibetans are still missing since a Chinese crackdown on the region after protests in March last year, a report by the International Campaign for Tibet said on Monday.
Security forces arrested thousands of Tibetans, often seizing them in the middle of the night on flimsy evidence of being "splittist" and tortured them, the new report said.
"There is still an intense climate of fear in Lhasa today," Kate Saunders, one the authors of the report, said. "(Tibetans) have made tremendous steps to show that they answer to the Dalai Lama, not the Chinese state."
The report was released a day before the Dalai Lama speaks on the 50th anniversary of the failed Tibetan uprising against Chinese rule, which led to the Tibetan spiritual leader's escape to northern India in 1959.
The Chinese Foreign Ministry and Public Security Ministry did not immediately comment, nor were they immediately able to provide a number for Tibetans detained but not formally charged.
Tibetans have staged more than 130 protests since major unrest began in the Tibetan plateau in March last year, the report said.
China regularly defends its rule of Tibet, saying it ended centuries of serfdom in 1959 and has since poured in development money and vastly improved the standard of life.
The Tibetan government-in-exile says more than 200 people were killed in the Chinese crackdown since last year in and around Lhasa.
The ICT, an activist organisation with ties to the exile Tibetan community, based its report on literature banned inside China and accounts of witnesses.
China's state-owned Xinhua news agency announced the opening of what it called "the first website dedicated to human rights in Tibet".
The website, in Chinese, English, German and French, features interviews with Tibetans but no Tibetan-language content.
A popular Chinese-language website focusing on Tibetan culture was shut last week for "upgrading and maintenance", while overseas groups said the webmaster of a Tibetan-language site also focusing on Tibetan culture was detained last week.