A state of maximum alert is in force around the Galeras volcano in the south-west of the country.
About 2,000 people from nearby villages have so far been taken to shelters.
The last major eruption from Galeras, one of Colombia's most active volcanoes, killed nine people in 1993, including scientists monitoring it.
An explosion of rock and gas from the volcano on Wednesday prompted the local authorities to step up plans to evacuate the surrounding areas.
Television pictures showed thick clouds of ash over the city of Pasto, whose 400,000 residents live about 8km (5 miles) from the volcano.
The Vulcanological Observatory in Pasto is monitoring the situation and latest reports from the area say the volcano's activity has now lessened.
The Galeras volcano, 520km (320 miles) south-west of the capital, Bogota, has a long history of activity.
Last November, it spewed ash that fell up to 50km (30 miles) away.Last Mod: 20 Eylül 2018, 18:16