A leading human rights group says eyewitness accounts confirm that Chinese forces used disproportionate force and deliberate brutality to put down protests two years ago in Tibet.

In a report released in New York late Wednesday, Human Rights Watch used interviews with more than 200 Tibetan refugees and visitors to create a detailed account of the March 2008 disturbances.

It says the Chinese authorities used undue force to break up the protests, which were followed by large-scale arbitrary arrests, brutal treatment of detainees and the torture of suspects in custody.

Human Rights Watch says the witness accounts document four instances in which security forces opened fire indiscriminately on protesters. At least 20 people were killed in the unrest.

China, which has long maintained that its forces acted in accordance with international standards and domestic laws, has pledged to handle all cases arising from the protests in an impartial manner.

But the report, titled "I Saw It With My Own Eyes," says thousands of Tibetans were detained without due process or regard to legal procedures. It says many violations continue today, including disappearances, wrongful convictions and the persecution of families.

In an accompanying press release, Human Rights Watch urges the Chinese government to investigate the handling of the protests and to open the region to media and international monitors.

The Chinese government sent in troops on March 14, 2008, to break up what began as peaceful protests by monks from monasteries in and around the Tibetan capital, Lhasa.

China said it was forced to act because government buildings and shops were being vandalized. Many Tibetans resent the growing presence of ethnic Han Chinese in Lhasa, where Han control many commercial enterprises.