Most of the animals had their skins shaven and one was found dead, frozen in liquid nitrogen and with signs of mutilation, according to reports carried by websites of the dailies Folha de Sao Paulo and O Estado de Sao Paulo.
Rat foetuses were also found during the operation which occurred at the Royal Institute lab located in Sao Roque, 60 kilometres from Sao Paulo. Police made no arrests.
The lab was not available for comment.
Ahead of their action, the activists went to police to complain of alleged mistreatment of animals at the lab, but they said no action was taken.
The lab cancelled a meeting with the protesters at the last minute and described the invasion as "an act of terrorism," insisting that its activities are monitored by Brazil's Health Surveillance Agency, according to O Estado.
Animal testing for scientific research is legal in Brazil and is regulated in line with international norms.