More than 230 people are dead after an "avalanche of water" engulfed the Colombian city of Mocoa, according to the Red Cross.
Hours of heavy rain caused several rivers to overflow and sent mud and sediment onto houses and roads in the city of about 350,000 people, authorities said.
Carlos Ivan Marquez, from Colombia's national disaster agency, said the river flooded around midnight on Friday/Saturday, destroying homes and killing many people in their sleep.
Witnesses said they had felt buildings vibrate as the flood began but there had been little time to escape.
More than 1,100 emergency workers and military personnel have been helping the rescue efforts in the city, which sits near Colombia's border with Ecuador.
Police commander Colonel Omar Bonilla told local radio station Caracol: "At this time we have removed 93 bodies - we have adults, women and infants."
President Juan Manuel Santos has arrived in the region, declaring a state of emergency and saying: "We will do everything we can to help (affected families).
"It breaks my heart."
President Santos said more than 200 people have been injured, 22 of them seriously.
Herman Granados, a surgeon at the local hospital, said that doctors were running out of blood to help treat patients.
He said: "Under the mud, I am sure there are many more (people).
There are also at least 100 people missing.
Mocoa's mayor Jose Antonio Castro said: "A big portion of the many houses were just taken by the avalanche, but above all the people were warned with enough time and they were able to get out, but houses in 17 neighbourhoods have basically been erased.
"The figures have been going up and in the crisis room they kept reporting more dead, we hope to God that it won't go up too much because it is very sad."
The force of the muddy water destroyed at least two bridges, ripped trees from their roots and lifted cars, carrying them downstream.