(CNN) -- Chinese forces broke international law during clashes with protesters across the Tibetan plateau in March 2008, a report issued Wednesday by Human Rights Watch concluded.
"It [the report] finds that the scale of human rights violations related to suppressing the protests was far greater than previously believed, and that Chinese forces broke international law -- including prohibitions against disproportionate use of force, torture and arbitrary detention, as well as the right to peaceful assembly -- despite government claims to the contrary," the group said.
The human rights organization said its 85-page report is based on 203 interviews conducted outside China with Tibetan refugees and temporary visitors between March 2008 and April 2010.
It cited Wu Shuangzhan, commander of China's People's Armed Police, who said on March 16, 2008, "None of the means ... adopted there have exceeded the constitutional rights of the armed forces or international law."
It then juxtaposed that statement with this one from Pema Lhakyi, a 24-year-old Lhasa resident who told the organization, "They were firing straight at people. They were coming from the direction of Jiangsu Lu firing at any Tibetans they saw, and many people had been killed."
The investigation found that violations continue, "including disappearances, wrongful convictions and imprisonment, persecution of families, and the targeting (of) people suspected of sympathizing with the protest movement."
The report called on China to investigate the protests and their aftermath, and to open the region to reporters and international monitors.
"It also needs to examine the conduct of its security forces, which eyewitnesses consistently say used disproportionate force; deliberately brutalized and mistreated Tibetans detained for suspected involvement in the unrest; and deprived detainees of minimum guarantees of due process of law, including formal notification of where, or why, they were held."
China has refused calls for an independent inquiry into the protests and their causes, and has tried to hide details of its related security operations, the report's summary said.
CNN was not able late Wednesday to reach anyone at the Chinese Foreign Ministry.
China has barred many news organizations from traveling freely in the region, allowing "only a handful of tightly scripted tours for select foreign media and diplomatic delegations" during the past two and a half years, the report said.
China also has refused to allow representatives of the International Committee of the Red Cross from visiting its prisons, it said.
The commander of the paramilitary People's Armed Police has said that security forces acted within the strictures of Chinese and international law.
"This characterization seems to be accurate in a few cases when security forces apparently exerted control when they faced large gatherings of Tibetan residents or monks," the report said. "At times, such groups posed genuine threats to public order, especially in Lhasa on March 14 and in several incidents where protesters targeted official buildings, police stations, vehicles, and Chinese-owned shops.
"But in most cases there is just too little information about the precise sequence of events to know if protesters became violent only after the police cracked down on peaceful protests or before the security forces intervened."
Human Rights Watch said it compiled official accounts and news reports of protests in at least 18 county-level areas in the Tibet Autonomous Region, and Qinghai, Gansu, and Sichuan provinces over two weeks.
"China's Xinhua state news agency acknowledges more than 150 incidents between March 10 and March 25. In Lhasa alone, 21 people were killed and several hundred injured during the March 14-15 time period according to government figures."
HRW called on China to release all Tibetan detainees who have not been charged or who were taken into custody "for exercising their right to peaceful expression" and to release details about those who were arrested on other charges or killed or wounded by security forces.
It also called on China to allow the ICRC access to prisons and places of detentions where Tibetans are held.
Finally, it called on China to hold accountable those responsible for using excessive force against unarmed protesters.