"They're trying to destroy the institution of democracy by killing us," said Oscar Nunez, director of the National Federation of Town Councils.
Authorities say up to 30 rebels from the Revolutionary Armed Forces of Colombia, or FARC, armed with grenades and assault rifles, barged into a town council meeting on Tuesday afternoon and killed six people, including five town councillors.
Bystanders and police officers also were hit by bullets and were being treated at area hospitals.
The attack appears to be a direct challenge to President Alvaro Uribe, who last year launched a US-supported military offensive in this region about 300km south of Bogota.
Troops hunting rebels
The offensive, called Plan Patriot, sent thousands of Colombian troops to hunt down FARC rebels in their jungle hideouts and remove them from small towns and villages where they held sway for years.
Plan Patriot has registered important successes, including the reduction of the FARC's size to 12,000 fighters from 16,000 one year ago. It has also helped establish a police presence in all the country's 1100 municipalities.
"What can we do when 20 or 30 guerrillas arrive to riddle them [council members] with bullets in their desks?"
Colombian Interior Minister
Three years ago, there were 200 municipalities with no government presence, many of them in this southern region.
But Tuesday's FARC attack showed the rebels are far from defeated in the south, their traditional stronghold. Government officials are wondering what else they could have done to protect the town council members and other civilians.
"What can we do when 20 or 30 guerrillas arrive to riddle them with bullets in their desks?" Interior Minister Sabas Pretelt asked after the attack.
The guerrillas travelled down a river in small boats to reach Puerto Rico, where they were picked up by fellow rebels in a pickup truck with an M-60 machine gun mounted in the back, said Colonel Gabriel Rodriguez, chief of the Caqueta state police force.
They sped to the centre of town to execute the attack, virtually unnoticed because they were in disguise.
"They were all dressed in Colombian army uniforms, and that created so much confusion," Rodriguez said as he surveyed the damage on Wednesday.
Nunez said FARC commanders offered their fighters $3400 for each council member they could kill.
The attack was the deadliest on politicians this year by the FARC, whose major attacks in recent months had been aimed primarily at police and the military.
Drop in attacks
Since Uribe took office three years ago, attacks on local town council members and small-town mayors have dropped significantly.
After 76 town council members were assassinated in 2002, just 18 were murdered in 2004.
"No matter what, I'm staying put. I'm not going to accept the lawlessness that (the rebels) want to provoke"
Councilman in Puerto Rico
But with Tuesday's killings, 16 have been assassinated this year.
Surviving town councilmen in Puerto Rico said they will not abandon their posts.
"No matter what, I'm staying put. I'm not going to accept the lawlessness that (the rebels) want to provoke," said Councilman William Lizcano, who along with his 6-year-old son, Adrian, escaped the bloodbath by jumping over a wall.
Puerto Rico, a town of 25,000, has been hit often by the FARC. Rebels assassinated the town's mayor in 2001 and then killed his replacement a few months later.
A third mayor was then chosen, and he barely escaped an assassination attempt in 2002 by the FARC that killed two bodyguards.
The FARC and a smaller rebel group have been fighting the Colombian government for 41 years.
The conflict, which also includes outlawed right-wing paramilitary groups, claims 3000 lives a year, most of them civilians.