The indigenous groups demanded the presence of a senior Brazilian official, saying that they wanted to start a new round of negotiations over the dam, Amazon Watch, an organization that works to protect the Amazon region and indigenous people, reported.

“Belo Monte will only succeed if we do nothing about it,” Juma Xipaia, an indigenous leader from the Xingu area, said in a statement released by Amazon Watch. “We will not be silent. We will shout out loud, and we will do it now.”

The move signaled a change in strategy by indigenous groups in their campaign to stop the dam. Legal challenges by local Amazonian communities — backed by international environmental groups like Amazon Watch — have done little to dissuade the government of President Dilma Rousseff to halt work on the dam, which would be the third largest in the world. Brazilian officials say the dam is badly needed to provide for future energy needs in growing cities like São Paulo.

“This is kind of a last resort,” said Atossa Soltani, the founder of Amazon Watch.

Ms. Soltani said indigenous groups were committed to nonviolent action. Last year, at a meeting at a village along the Xingu River, which the movie director James Cameron attended, about 70 indigenous leaders vowed to form a new tribe of 2,500 to occupy the construction site and, if necessary, sacrifice their lives to defend their native lands.

Environmentalists say that the $11 billion dam would flood about 200 square miles of the Amazon region and dry up a 60-mile stretch of the Xingu River, affecting fishing and the indigenous groups’ way of life.