The new law passed yesterday stipulates that killers of police officers, prison officers or members of the armed forces will be sentenced to up to 30 years in prison, against a maximum 20 before.
After the vote, president of the Senate, Renan Calheiros, said there had been "strong demand from society" for the stiffer penalty.
The new law, which needs to be signed off by President Dilma Rousseff, also toughens punishment for those who kill the relatives of law enforcement officials.
But the group says that police on average kill six people every day.