They were the latest victims of floods that have struck the Pacific side of South America over recent months, also killing scores of people in Peru and Ecuador.
In the southwestern Colombian town of Mocoa, the surge swept away houses, bridges, vehicles and trees, leaving piles of wrecked timber and brown mud, army images from the area showed.
The mudslides struck late Friday after days of torrential rain in the Amazon basin area town of 40,000.
"The latest information we have is that there are 206 people confirmed dead, 202 injured, 220 missing, 17 neighborhoods hit hard," Colombian Red Cross chief Cesar Uruena told AFP.
President Juan Manuel Santos visited Mocoa, the capital of Putumayo department, on Saturday to supervise rescue efforts in the heavily forested region.
He declared a public health and safety emergency to speed up rescue and aid operations. He also expressed his condolences to victims' families.
Nation in mourning
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.
The rivers flooded causing a "big avalanche," the army said in a statement.
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."
The authorities activated a crisis group including local officials, military personnel, police and rescuers to search for missing people and begin removing mountains of debris, Marquez said.
A thousand emergency personnel were helping the rescue effort. Mocoa was left without power or running water; there were reports of some looting in efforts to get 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.
"People do not know what to do... there were no preparations" for such a disaster, he said.
"We are just starting to realize what has hit us."
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 at the time.
A landslide the month before killed 10 people in the north of the country.
Climate change can play a big role in the scale of natural disasters, such as this one, 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.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)