Al Jazeera's Gabriel Elizondo said that as the Eletrobras engineer spoke, participants booed while some complained his comments were antagonising and patronising.
After he sat down, tribal members surrounded him and began a ritualistic war dance.
The engineer, Paulo Fernando Rezende, was rushed out of the building after a melee formed and he fell from his chair.
It was not immediately clear whether Rezende was intentionally slashed or received a cut to the shoulder when he was surrounded and pushed to the floor.
Police said they were still investigating and that no one was in custody
"He's lucky he's still alive," Partyk Kayapo, told Associated Press.
"They want to make a dam and now they know they shouldn't."
Following the attack, Kayapo and dozens more members of his tribe danced
with their machetes.
The Brazilian government said the proposed US$6.7 billion hydroelectric dam on the Xingu River, which flows into the Amazon, will supply Brazil with an estimated 11,000 megawatts of power and is essential to meet growing energy demand.
If the plan goes ahead, the Belo Monte project will be the world's third largest hydro-electric dam.
The attack recalls a similar meeting in 1989 when Indians held a machete
to the face of another Eletrobras engineer during protests against a series of proposed hydroelectric dams on the Xingu river.
Following that incident, the World Bank canceled loans to Brazil for the dam.
Luiz Inacio Lula da Silva, Brazil's president, said earlier this month that his country's pro-environmental policies have not changed.
But critics point to his decision to disband the environmental protection agency Ibama in recent months as a sign of placing priority on economic-friendly projects before ecological concerns.