Police and emergency officials were on high alert after the 14,110-foot Galeras volcano became active at dawn and dumped heaps of ash on the city of Pasto, 12 miles away.
"It was a brief eruption of ash for 30 minutes that was not preceded by a temblor inside the volcano," said Marta Lucia Calvache of Colombia's Volcanology Institute. "But there is still a thin plume of ash leaving the crater, and we can't rule out the possibility of further eruptions."
The government this month ordered the preventive evacuation of thousands of people living in the shadow of the volcano amid signs of an imminent eruption. But many farmers are believed to have defied the order and stayed behind, fearful of losing their livelihoods by leaving crops unattended.
Calvache urged any families who remain in a wide area surrounding the volcano to leave immediately and seek medical treatment if they have trouble breathing.
"A lot of ash has fallen. We are scooping it up and putting it into plastic bags. There is a strong smell of sulfur in the air," said Yolanda Casas, a Pasto resident wearing protective goggles and a face mask.
Schools and many offices in Pasto were closed for the day.
The Galeras has a long history of activity, fraying nerves in Pasto. More than 100 minor tremors were felt in the city during the volcano's last major eruption, in April 2002, although no damage or injury was reported.
A 1993 eruption killed nine people, including five scientists from around the globe who had descended into the crater to sample gases at the moment it blew.