Protesters celebrated the rejection of PEC 37, which would have assigned the power to conduct criminal investigations solely to the police, not to federal prosecutors, the BBC reported.
The proposed amendment was criticized because it could have opened the way for corruption in the country, the report said.
Protests began two weeks ago over higher bus fares but then were focused on government corruption and the cost of next year's World Cup soccer tournament.
On Tuesday, Brazilian President Dilma Rousseff proposed a series of reforms aimed at quelling the protests.
Among them was a promise to improve healthcare and education.
Congress also voted to funnel royalties from newly discovered oil fields into healthcare and education, the BBC reported.
Legislators have promised to next vote on proposals to make public transportation free for students and to make corruption a heinous crime, the BBC said.