The bomb was detonated at around 1500 GMT in the town of Puerto Rico, just as a boat carrying 56 people was docking, army general Luis Eduardo Barboza told AFP.
An unidentified woman was carrying the bomb, he said. She died in the blast.
“It was horrendous,” Munoz told AFP. “There are many body parts, not all the people could be identified. We're working to see if there are more than six dead.”
The Revolutionary Armed Forces of Colombia (FARC) is Latin America's oldest and largest rebel force.
It has been fighting a long-running battle to topple the government in Bogota and is infamous for its kidnapping, drug-running and bomb attacks on civilian targets.
Meanwhile, the rebel group's second-in-command Raul Reyes said Sunday in an interview at his mountain hideout that the United Nations had responded to his request for mediation between the guerrillas and the Colombian government.
He said last week that he was prepared to travel to New York to meet with UN Secretary General Kofi Annan to facilitate peace talks.
The Colombian government of President Alvaro Uribe has also shown its willingness for the UN to play a role in talks between the two sides.