Medellin's mayor appealed to motorists to stay off roads to facilitate the arrival of rescuers, ambulances and first responders.
The President of Colombia, Juan Manuel Santos, tweeted: "Faced with emergency in Guatape the air force and relief agencies are taking care of the situation.
"We are ready to provide required assistance."
"The most important thing right now is to save lives," said Mayor Federico Gutierrez.
Dramatic videos circulating on social media show the ferry, called El Almirante, rocking back and forth as people crawled down from a third-floor roof as the ship began sinking. A flotilla of recreational boats rushed to the scene to pull people from the vessel, avoiding an even deadlier tragedy.
It's unclear what caused the boat to sink but survivors told local media that it appeared to be overloaded and none of the passengers on board were wearing life vests.
Daniel Giraldo, owner of an Italian restaurant in Guatape, said he went to the bay after hearing the sound of ambulances. When he got to the shore people told him the ship had gone under.
"It sank in a matter of four minutes," he said.
Next he went to the hospital where he said he saw a baby girl in a wet dress who had been saved but whose mother he was told is missing.
The reservoir surrounding the soaring rocky outcrop of El Penol is a popular weekend destination a little more than an hour from Medellin. It was especially busy on Sunday as Colombians celebrated the long holiday weekend.
The reservoir, which was created as a result of a controversial dam built in the late 1970s, is widely recommended as a place to visit, with boat rides on the water being cited.
- AP, Daily Telegraph UK
