The surge swept away houses, bridges, vehicles and trees, leaving piles of wrecked timber and brown mud, army photographs from the town of Mocoa showed.
The mudslides struck late Friday after days of torrential rain.
President Juan Manuel Santos visited Mocoa, the capital of Putumayo department, on Saturday to supervise rescue and assistance efforts in the heavily forested region.
"I have just been informed that 112 people are dead," he said. "We don't know how many more there will be; we are still searching."
He warned that the death toll -- initially put at 16 -- would probably rise further because 200 people were still missing.
"The number is rising enormously and at considerable speed," he said.
The disaster is of "large proportions," he added.
Putumayo Governor Sorrel Aroca called the development "an unprecedented tragedy" for the area.
There are "hundreds of families we have not yet found and whole neighborhoods have disappeared," he told W Radio.
Carlos Ivan Marquez, director of the National Disaster Risk Management Unit, told AFP the mudslides were caused by the rise of the Mocoa River and tributaries.
Some 130 millimeters (5 inches) of rain fell Friday night, Santos said. "That means 30 percent of monthly rainfall fell last night, which precipitated a sudden rise of several rivers," he said.
He promised earlier on Twitter to "guarantee assistance to the victims of this tragedy, which has Colombians in mourning."
"Our prayers are with the victims and those affected," he added.
The authorities activated a crisis group including more than 100 local officials, military personnel, police and rescuers to search for missing people and begin removing mountains of debris, Marquez said.
"There are lots of people in the streets, lots of people displaced and many houses have collapsed," retired Mocoa resident Hernando Rodriguez, 69, said by telephone.
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