This definitely true in many part of the world. Out of all forces, police have most contacts with the criminal elements in the society and it is logical for these criminals to want to bribe or force their way out of their charges or crimes.
With police been underpayed is some part of the world, they are always surely tempted to twist the law with some incentives from the criminals. Not justifying any police receiving bribes, been racist or overzealous but they are also human.
Again about been human, some particular race or set of people in the society are more noted or stigmatized to commit crimes than others. The police in such society will likely be more overzealous or use high-handedness against an individual from such that race or set. Also police are use as instrument to punish some people by the influentials.
All these are not right as every case should be treated with pure objectivity regardless of tribe, race, political difference and so on. But I guess this is the world we live...so much injustices everywhere.