The incident was witnessed by an ethnic Chinese woman, who confirmed the arrest of four Tibetans last Sunday. According to the International Campaign for Tibet, which spoke to the woman, Tibetan protesters met outside the Amdo Cement plant in an attempt to submit a petition letter to the authorities demanding greater pollution controls.
As soon as the gathering was formed, anti-riot police moved it and opened fire on them, wounding 15.
According to the South China Morning Post, the petition demanded tougher control of the dust coming from the factory. It also called on the factory to withdraw from a religious site and an old road that it forcibly occupied.
The cement factory opened in 1985 as a state-owned enterprise, but privatised in 1998. It has since become one of Gansu's major cement makers.
Villagers complained that after the factory increased production last year, their lives were negatively affected.
“For those who live near the cement factory, agricultural yields have dropped significantly, by more than 60 per cent, because of the huge amount of dust emitted by the factory," the letter said.
“The cows and sheep no longer eat the grass growing on contaminated slopes, forcing us to buy pasturage somewhere else. Our natural pasture has lost its function to feed the livestock. This has significantly affected our life," the letter added.
In the meantime, the central government has stepped up its repression in Tibet and Tibetan-populated areas since deadly protests broke out in the spring of 2008.
Tighter security measures have not however stopped protests from becoming more frequent.
One of the government’s latest steps has surprised Chinese analysts. Beijing has in fact ordered a crackdown on printing and photocopying documents in Tibetan. For the government, this measure was necessary to "prevent illegal activities”.
Han Chinese experts on Tibetan affairs have complained about the move, which in their view can only fuel tensions and invite further resistance in the region.