Cameron and Weaver are joining hundreds of demonstrators Monday, urging the government to halt the planned Belo Monte project.
Brazil's government says Belo Monte will provide much-needed clean energy for the country.
But Indian groups say they will be displaced by the dam. Environmentalists say its benefits won't make up for the damage to the jungle.
Cameron was in the Amazon last month with former U.S. Vice President Al Gore, pushing for a halt to rain forest destruction.
The Associated Press contributed to this report.
