The tribute was unveiled the day the activist would turn 43 years old. The monument was placed where she used to gather with voters to talk about achievements.
Prosecutors argue Pereira and Phillips were ambushed and killed after Pereira asked Phillips to photograph the boat belonging to the defendants in order to document the illicit practices.
The Brazilian president announced he had decreed official mourning in the country, in recognition of the solid ties between the Japanese politician and Brazil.
The observatory should be in charge of monitoring violence against Congolese citizens in Brazil, especially the case of Moïse Mugenyi Kabagambe, murdered in Rio de Janeiro.