Police in Beijing briefly detained three foreign Tibet activists after they unfurled a banner denouncing China's new railway line to Tibet.
Protesters at the site Friday say the activists from the United States, Canada and Britain, were detained for about three hours after they demonstrated at the capital's main train station.
Their protest banner read "China's Tibet Railway, Designed to Destroy."
China will open the 11-hundred kilometer railway Saturday, linking Golmud, in Qinghai Province, with Lhasa, in Tibet.
It will be the world's highest altitude rail line.
Critics say it will harm Tibet's environment by opening its timber and mineral resources to further exploitation.
Chinese officials say the railway will open new markets for Tibetan products and boost its tourism industry.
Tibet has been ruled by China since its troops invaded the region in 1950. Beijing says China has a history of sovereignty over Tibet that goes back several centuries.