Tibetan exiles say scores were killed by Chinese troops in March
More than 1,000 Tibetans detained during protests against the Chinese government in March remain unaccounted for, Amnesty International says.
In a report, the human rights group said there were reports that detainees had been beaten and deprived of food.
Ahead of the Olympic torch relay through Tibet, Amnesty asked China to "shine some light" on the situation.
China says rioters killed at least 19 people. Tibetan exiles say security forces killed dozens of people.
The anti-China protests led by Buddhist monks - the worst in the region in 20 years - began in Lhasa on 10 March.
After the riots, pro-Tibetan protesters threw China's global Olympic torch relay into disarray as it passed through several cities, including London, Paris and San Francisco.
Hundreds of raids
Olympic organisers in Beijing have confirmed that the torch will reach Tibet on Saturday.
A planned three-day stay there has been cut to one day because of schedule adjustments linked to last month's Sichuan earthquake in China, they said.
With the torch relay about to enter Tibetan areas, this should be an opportunity to shine some light on the situation there
The relay will run through the main city, Lhasa, the centre of the anti-China protests.
Announcing the update report, Amnesty International's Asia-Pacific Director Sam Zarifi said the information coming out of Tibet painted "a dire picture of arbitrary detentions and abuse of detainees".
Official reports only provide information on a small number of those who have been sentenced after questionable trials, Amnesty said.
It said that , with foreign journalists still not allowed into Tibet, reports coming through friends and family members to the media and Tibetan organisations suggested that police had carried out hundreds of raids on monasteries, nunneries and private homes.
Tibet: Protests began in Lhasa on 10 March, dozens reported dead over weekend
Gansu: Unrest spread to Machu, where Tibetan government in exile says 19 died, and near Hezuo, where protesters were filmed tearing down Tibetan flag
Sichuan: Reports of riots and casualties in Aba, and of unrest in other locations
Qinghai: Other unrest reported
"Those who dare to find ways of sending information to foreign media or human rights organisations regarding protests and arrests, risk arrest and imprisonment," Amnesty said.
"With the torch relay about to enter Tibetan areas, this should be an opportunity to shine some light on the situation there," Amnesty's Sam Zarifi said.
Tibet's spiritual leader, the Dalai Lama, has appealed for Tibetans not to protest during the Olympic Torch visit to the region.
The leader of Tibet's government-in-exile said recently he was fully supportive of the Games beginning in August, and therefore the torch.
Beijing says the Dalai Lama incited the March violence. He denies this and accuses the government of human rights abuses.