The country’s Institute of Geology and Mining had warned the nearly 8,000 nearby residents that the mountain was about to erupt.
But many refused to leave and hunkered down in their homes, according to officials.
Galeras has produced similar activity in recent years, and those living near it say they now mainly ignore the regular warnings of an impending eruption.
But the government still tries to impress the potential danger Galeras poses.
“People are obliged to leave, no one can sit around debating over what might occur, even if up until now nothing has happened, we have to remain alert,” said Interior Minister German Vargas Lleras.
He called on people who could be affected by lava flows to "urgently" move to temporary shelters established by the state in the municipalities of Pasto, La Florida, and Nariño.
The volcano last erupted in January, triggering forest fires on its slopes.