The Baram dam is the latest in a series of controversial hydroelectric mega-dams planned by the Sarawak government as it pushes economic development in one of Malaysia's poorest states.
Indigenous Kenyah, Kayan and Penan people began blocking the main entry road to the dam's location and the site where the dam's developer, state-owned Sarawak Energy (SEB), had stored its heavy machinery last afternoon, according to NGO Save Sarawak's Rivers Network.
The group's vice-chairman Raymond Abin told AFP today the blockade was still going on.
"The call is to stop the project and all activities related to the construction of the dam because SEB is already starting soil investigation," he said.
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The building spree of hydroelectric dams has been dogged by controversy as activists allege massive corruption while indigenous people complain it has flooded rainforests and uprooted tens of thousands of people.
But the government of resource-rich Sarawak says it hopes a plentiful supply of hydropower from the state's powerful jungle rivers will attract new industries.
Sarawak Energy has insisted that displaced villagers are being compensated fairly. It could not be reached for immediate comment on the Baram blockade.
Swiss-based activists at the Bruno Manser Fund, which has repeatedly accused Sarawak's longtime chief minister Taib Mahmud of corruption, said that the protests would add to scrutiny on Malaysia's human rights record.
Sarawak tribespeople have staged increasingly frequent protests and road blockades in recent years over the dams.