The police were driving through mountains in the municipal area of Riofrio, about 160 miles (250km) southwest of the capital Bogota, on Monday night when the bomb exploded, police said on Tuesday.
Police sources blamed suspected Marxist rebels for the attack.
The 17,000-strong Revolutionary Armed Forces of Colombia, or FARC, the country's largest rebel army, is active in the area.
The incident appears to be the second major FARC attack on security forces in two weeks, shattering a relatively long period of inactivity.
In July, a large group of FARC rebels shot and killed 13 out of a group of heavily outnumbered soldiers who were trying to defend a bridge in the southern jungle province of Putumayo.
President Alvaro Uribe, supported by US military aid, has increased defence spending and put the armed forces on the offensive, leading some analysts to suggest the guerrillas have been pushed into retreat.
The rebels have been fighting for socialist revolution for 40 years in a conflict which has claimed thousands of lives.
FARC allegedly fund their movement by demanding money from cocaine traffickers and kidnapping for ransom.