China says 22 people died in the riots, but overseas Tibet supporters say many more were killed in the crackdown.
Human Rights Watch said its researchers interviewed 203 Tibetan refugees and visitors outside China between March 2008 and April 2010.
The report features rare eyewitness accounts detailing excesses in the unrest.
Witnesses were quoted as saying that on March 14, 2008, security forces in Lhasa opened fire on protesters near the Barkhor Sqaure, the heart of the old city.
At several rallies, security forces also hit demonstrators with batons and rifle butts until they were no longer moving, the New York-based rights body says.
As protests spread across the Tibetan plateau, security forces shot at secondary school students headed to a demonstration and at monks and civilians marching toward government buildings.
"When the soldiers showed up later, they threw tear gas. A gas canister hit my leg and I couldn't walk any more"
The report cites a 24-year-old Tibetan woman who was near the Barkhor Square and said protesters roamed freely on March 14 until the afternoon, when troops showed up and opened fire.
"When the soldiers showed up later, they threw tear gas. A gas canister hit my leg and I couldn't walk any more," the report quoted the woman as saying.
"Then there was indiscriminate shooting and we saw two people shot dead in front of us."
A 33-year-old monk from a monastery west of Lhasa said he was beaten with clubs and sticks by guards at detention facilities where he was held, and beaten again, with sand-filled rubber tubes, when sentenced to a year in a labour camp.
Nicholas Bequelin, Asia researcher for the group, said: "Over the past two years, security forces acted in a way that is completely disproportionate to the actual threat to public order.
"The Chinese government could do something about it. This is not about their sovereignty in Tibet, this is about how their security forces behave."
The unrest started with several days of anti-government protests by Buddhist monks in Lhasa, before spreading to communities across China's west. Tibetans attacked Chinese-owned shops and homes.
China has Beijing has sought to quash accounts of rights abuses, flooding the region with troops and has put Tibetans under tighter scrutiny, reduced the flow of international tourists and allowed in only a few foreign reporters under government escort.
The Chinese government also criticised the the Human Rights Watch report, accusing it of being bias.
Qin Gang, the Chinese foreign ministry spokesman, said in a statement that the events in Lhasa were "serious, violent criminal incidents that caused great loss to the lives and property of the local people".
He said the authorities enforced the law in a legal, civilised way and that ethnic customs and human dignity were respected.