Liberia's police often act as "predators" who rob people of money and goods rather than protecting them, says campaign group Human Rights Watch.
Street vendors and taxi drivers are the main victims of police criminality, its report, No Money, No Justice, says.
Liberia is recovering from a brutal civil war that ended in 2003.
President Ellen Johnson Sirleaf, a Nobel peace laureate, declared corruption "public enemy number one" when she took office in 2006.
The government has not yet commented on the report by the US-based rights group.
Liberia's police and army are expected to take a greater responsibility for security as the United Nations reduces its 18,000-strong force in Liberia to 3,750 by 2015.