Army experts believe the device was detonated remotely
At least seven people, including a baby, have been killed in an explosion in Colombia, according to military officials.
An army spokesman said a bomb went off as about 60 people were getting off a boat on the river Ariari, in the central town of Puerto Rico, 125 miles (200 km) southeast of Bogota.
He said the army believes that one of the seven who died was a woman who was carrying the explosive device in a box, but there are no indications that she was a suicide bomber.
Thirty-eight other people were injured by the blast, which the army blamed on the Revolutionary Armed Forces of Colombia (FARC) - the country's largest rebel group - who also launched a ground attack on the town as security forces moved in to evacuate the wounded.
The BBC's Jeremy McDermott, in Medellin, said the bomb was believed to have been a proxy device set off by remote control when the rebels saw the boat approach the dockside.
Army experts believe the woman did not know that she had the device with her.
Our correspondent said the rebels have used proxy bombs before - forcing people to drive cars filled with explosives at army checkpoints before detonating the devices by remote control.
A senior local official, Wilson Munoz, said the worst-injured were to be transported to hospitals by aircraft.
He blamed FARC for the bombing saying the group "doesn't respect any parameters of human rights".
"It was horrendous," he said. "There are many body parts, not all the people could be identified. We're working to see if there are more than six dead."
BBC Mundo said the incident happened a day after a politician in the north-eastern province of Segovia was seriously injured when he was shot by two men on a motorbike.
Colombians are concerned about an increase in the level of violence in the run up to 26 October, when elections are to be held for mayors and councillors in around 1,100 municipalities and governors in 32 departments of the country.
On Sunday the Brazilian newspaper Folha de Sao Paulo published an interview carried out last week with Raul Reyes, one of the FARC leaders in the Colombian Amazon.
He said the group was not worried about military pressure from Colombian President Alvaro Uribe.
"Uribe only has three short years left in power and FARC have all the time in the world," he said.
"It's been 39 years of fighting. We will take all the time necessary to reach our objectives."
Under Mr Uribe, the Colombian Army, backed by $3bn of US aid in recent years, has taken the offensive against the rebels.
The 17,000-strong FARC, unable to take the military on head-on, has increasingly resorted to setting off bombs in urban centres.