A bomb exploded outside a police station in the Pacific port city of Tumaco just as lunch hour ended, killing at six people and wounding 71, authorities said.
The bomb appeared to be a motorcycle packed with explosives, Tumaco security chief Hernando Cortes told The Associated Press.
Hernando Cortes, a city official, said the blast killed three police officers and three civilians yesterday. He said the 71 injured included 35 police officers.
Gen Rodolfo Palomino, the national police director of citizen security, blamed the leftist Revolutionary Armed Forces of Colombia, or FARC, for the attack, though Tumaco's mayor, Victor Gallo, refrained from assigning blame.
President Juan Manuel Santos condemned the attack, calling it an "act of desperation."
The southwestern port is one of Colombia's most lawless cities, a cocaine-smuggling hub where leftist rebels, right-wing criminal bands and drug traffickers are all present.
The 1:58 pm (local time) blast occurred "in an area with a great abundance of population, as much for the hour as the geographical location," Palomino said. It is the end of lunch hour in Colombia.
Tumaco is in Narino state, which borders Ecuador and is laced with dozens of rivers popular with drug traffickers, who include the FARC.
The most recent major FARC attack in the region occurred in October when 10 soldiers were killed in a mortar attack while on patrol in a rural part of Tumaco.