By Richard Collings
BBC News, Pasto, Colombia
One of Colombia's largest volcanoes has erupted, spewing huge amounts of ash and sulphur fumes.
Army troops are co-ordinating the evacuation
Pasto, the city that lies immediately below the Galeras volcano, is home to nearly half a million people.
The evacuation of the area began days ago, but many farmers are still refusing to vacate their homes.
The volcano, part of the Andes mountains, last erupted 12 years ago, killing nine people, including scientists who were monitoring it.
The compulsory evacuation order covers a 15km (9-mile) radius, including one of the most affluent areas of the city.
Army troops have been drafted in to co-ordinate the evacuation.
Local farmers fear their land and livestock will be taken by others if they leave. People here are suspicious that they will not get their homes back once the emergency has passed.
President Alvaro Uribe has appealed to everyone to leave immediately.
"Don't worry, the government will pay for your permanent relocation, but it might take some time," he said.
Workers have been cleaning up tons of ash that settled in the streets after the first eruption took place in the early hours of Thursday morning.
There is a very strong smell of sulphur in the air.
The local authorities have closed all schools in the area and water supplies are being rationed.
They have ordered taxi and bus drivers to stop profiteering from the crisis - customers have been charged three times more than usual.
And pharmacies have also been doing very well, selling the face masks that everyone needs at extortionate prices.
It is 20 years since one of Colombia's other large volcanoes, the Nevado del Ruiz, erupted. That caused a massive landslide which killed 25,000 people.