Santos, who travelled to the southern town to personally oversee relief operations, warned the toll could keep climbing.
“Unfortunately, these are still preliminary figures,” he wrote on Twitter.
“We offer our prayers for all of them. We send our condolences and the entire country’s sympathies to their families.”
Survivors described gruesome scenes in the remote southern town, as rescuers kept up a bleak search for victims in the muck and debris.
Covered in mud, 38-year-old Marta Gomez told of going to search for her missing niece – and making a chilling find instead.
“I went to look for my niece, but I couldn’t find her. I dug and dug and found what turned out to be a baby’s hand. It was horrible,” she said in a shelter set up for the newly homeless.
As she stood in line waiting to register for government assistance for those who lost their houses, shesaid she had given up on finding her niece.
“The mud took her away. I’ll never see her again,” she said, clinging to the leash of her equally muddy German shepherd.
Rescuers worked in stifling heat under a cloudy sky in the remote Amazon town, the capital of Putumayo department.
The debris left by the mudslides was everywhere: buried cars, uprooted trees, children’s toys and stray shoes sticking up out of the mud.
The torrent of mud, boulders and debris struck the town with little warning late Friday (Mar 31) after days of heavy rains that caused three area rivers to flood.
It swept away homes, bridges, vehicles and trees, leaving piles of wrecked timber.
Most of the hardest-hit neighbourhoods in the town of 40,000 are poor and populated with people uprooted during Colombia’s five-decade-long civil war.
A “profoundly saddened” Pope Francis said he was praying for the victims.
Santos declared an emergency to speed up aid operations.
Health authorities said they had dispatched sanitation specialists in hopes of preventing outbreaks of disease.
An unexpected offer of help also came from the Revolutionary Armed Forces of Colombia (FARC), a leftist rebel group engaged in a historic peace process with the government.
It said FARC members were prepared to help rebuild the town.
Marta Ceballos, a 44-year-old street vendor, said she lost everything, but is thankful her family is alive.
“Dear God, I don’t want to even remember that,” she said.
“To see how some people screamed, and others cried, ran, tried to flee in cars, on motorcycles, and how they were trapped in the mud. It’s all too, too difficult.
“The only things I fortunately did not lose were my husband, my daughters and my nephews,” she said.
Electricity and running water have yet to be restored to Mocoa. Local authorities said repairing the electrical substation would take time.
There were reports of people looting stores searching for bottled water.
“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.
“There were no preparations” for such a disaster, he said.
Several deadly landslides have struck Colombia in recent months.
A landslide in November killed nine people in the rural southwestern town of El Tambo, officials said.
The previous month, 10 people lost their lives in a mudslide in the north of the country.
The Pacific rim of South America has been hard hit in recent months by floods and mudslides, with scores killed in Peru and Ecuador.
Climate change can play a big role in the scale of such natural disasters, a senior UN official said.
“Climate change is generating dynamics and we see the tremendous results in terms of intensity, frequency and magnitude of these natural effects, as we have just seen in Mocoa,” said Martin Santiago, UN chief for Colombia.