Police last week opened fire on locals in Kardze, a Tibetan-majority area of China's southwestern Sichuan province, rights groups and US-funded broadcaster Radio Free Asia (RFA) said, citing local sources.
Three people aged from 18 to 60 are now confirmed dead from injuries inflicted during the shootings, British-based group Free Tibet said in a statement, without specifying how the other two are believed to have died.
The US-based International Campaign for Tibet said one protester had committed suicide in custody.
China's ethnic minority regions in Tibet and far-western Xinjiang, home to the mainly Muslim Uighurs, have been regularly hit by unrest in recent years.
Rights groups blame the clashes on cultural and religious repression, claims the Chinese government denies.
The protests in Kardze were sparked by the arrest of a local leader, reports said.
The International Campaign for Tibet said police had fired "anti-riot projectiles".
Free Tibet said several shooting victims were denied medical treatment.
"This shooting and the subsequent treatment of detainees exposes the reality of China's so-called 'rule of law' in Tibet," the group added.
China heavily restricts local and foreign media from reporting in minority areas, making it very difficult to independently verify such reports of unrest.
In recent years Tibetans have turned to self-immolation to protest against Beijing's rule.
At least 120 Tibetans in China have set themselves alight since 2009, according to tallies kept by Free Tibet and RFA.
Kardze, known in Chinese as Ganzi prefecture, has in the past been a flashpoint for protests and was the site of a self-immolation by a nun in March, Free Tibet and RFA reported.
Beijing says it has brought economic development to poverty-stricken Tibetan areas, while claiming to grant broad religious freedoms.