The disaster took place around 3 am local time (0800 GMT) in the town of Salgar, about 60 miles (100 kilometers) southwest of Medellin.
Residents described being stirred from bed by a loud rumble and neighbors' shouts of "The river! The river!" as an avalanche of heavy debris and mud sent several homes and bridges plunging into the steep Libordiana ravine. Those lucky enough to survive barely had enough time to gather their loved ones.
Disaster relief authorities supported by police helicopters are evacuating residents near the ravine for fear of another mudslide.
Gen Jose Angel Mendoza, the top police officer in the area, told Blu Radio that 47 people were confirmed dead but that the number of casualties is likely to rise. He said another 25 had been injured and an unknown number of people are still unaccounted for.
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Authorities are calling on volunteers to send water, food supplies and blankets to stem what they described as humanitarian emergency.
The town of 18,000 lies amid one of Colombia's major coffee-growing regions. Former President Alvaro Uribe, who spent part of his childhood in Salgar, announced on Twitter he was heading to the town to accompany the victims.
He'll be joined by President Juan Manual Santos, who Mendoza said was also on his way to the town to oversee disaster relief operations.