A bomb exploded in front of a police station in Libya's eastern city of Benghazi on Wednesday, wounding four officers, the force said.
Libya's government is struggling to contain fighters and militias who gained power during the revolt that started in Benghazi and went on to oust Muammar Gaddafi last year.
A police official said it looked like the bomb had been buried in the ground outside the police station in the al-Fuyahat area of the city and went off when a car approached.
"The two policemen inside the car were severely injured, and two others who were standing at the entrance of the building were also wounded," the official said.
The U.S. ambassador to Libya, Christopher Stevens, was killed in the city after Islamist gunmen attacked the U.S. consulate and a safe house refuge in September, in the worst of a string of attacks on international convoys and official buildings.
The head of police in Benghazi was killed in front of his home last month.