President Santos arrived at the disaster zone on Saturday and warned the death toll could rise even further as the search for survivors continues.
The incident triggered by torrential rain happened around midnight in Mocoa, a city of 350,000 located near Colombia’s border with Ecuador.
A surgeon at the local hospital said he believes there are at least 300 people injured and that doctors are running out of blood.
Witnesses described feeling buildings vibrate and said there was little time to seek refuge. Muddy water and debris quickly surged the city’s streets, toppling homes, ripping trees from their roots, lifting cars and lorries and carrying them downstream.
With most of the community asleep when the water avalanche began, many residents did not have enough time to climb on top of their roofs or seek safety on higher ground.
Surgeon Herman Granados, at the local hospital, said: “Under the mud, I am sure there are many more.”
Earlier President Santos said in a statement: “Our prayers are with the victims.”
Witnesses described feeling buildings vibrate as the flood began. Although an alarm reportedly went off, it could not be heard throughout the city, survivors said.
Videos that some residents posted online showed vast areas filled with wood planks and debris. People could be heard calling out the names of missing loved ones.
“There are many people looking for their relatives,” said Oscar Forero, a spokesman with the Colombian Red Cross.
The Red Cross planned to set up a special unit in Mocoa on Saturday afternoon to help relatives search for their relatives.