At least 60 Tibetans were injured after Chinese police fired into a crowd of protesters, rights group Free Tibet and US-based Radio Free Asia report.
The shooting occurred on Sunday in Biru county, as villagers demanded police free a man who led separate protests in September, the reports said.
There had been clashes in September after Tibetans refused to fly China's flag outside their homes, reports said.
Tibet is governed as an autonomous region in China.
China has been widely accused of repressing political and religious freedoms in Tibet. Beijing rejects this and says economic development has improved Tibetans' lives.
A Chinese Foreign Ministry spokeswoman said that she was unaware of the reports, and declined to comment further, Reuters news agency said.
A policeman at the public security bureau in Tibet's Biru county also told AFP news agency that there was "no protest, no one injured".
Foreign media are only allowed to enter Tibet at the invitation of the Chinese government. These visits are rare and tightly controlled, making it almost impossible to independently verify reports.
On 6 October, a crowd of protesters gathered outside the local government office appealing for the release of local Dorjee Dagtsel, Radio Free Asia, and UK-based Free Tibet, said.
Security forces opened fire into the crowd, injuring at least 60 and leaving two in a critical condition, they said.
Free Tibet added in the statement that the forces "deployed tear gas" and beat the protesters.
However, the International Campaign for Tibet said it was not clear if the troops fired live rounds or tear gas.
Free Tibet named the two critically injured Tibetans as Tagyal, who had a broken femur, and Tsewang, who was shot in the jaw. It said they had been taken to a hospital in the Tibetan capital, Lhasa.
All the phone lines in the region have been disconnected, exile Tibetan web portal Phayul reported.
The incident appears to be latest sign of the continuing unrest on China's Tibetan plateau, the BBC's Martin Patience in Beijing reports.
Since serious ethnic rioting broke out in 2008, more than 100 Tibetans have set themselves on fire in protest against what they say is Beijing's repressive rule, our correspondent adds.
Beijing claims a centuries-old sovereignty over the Himalayan region, and says Tibet has developed considerably under its rule.