The Brazilian President Dilma Rouseff has inaugurated a truth commission to investigate human rights abuses from 1946-1988, including those committed during military rule.
However, an amnesty law, passed by the military regime in 1979, means that no one can face prosecution for crimes committed during military rule.
"Brazil deserves the truth, new generations deserve the truth, and - above all - those who lost friends and relatives and who continue to suffer as if they were dying again each day deserve the truth," President Rousseff said.
The UN High Commissioner for Human Rights, Navi Pillay, said the commission is "a necessary and very hopeful step".
Over 500 people are thought to have been killed or disappeared in Brazil during the years under military rule. Thousands were tortured, exiled or deprived of their political rights.
