A volcano in Ecuador that has not erupted in nearly 140 years is belching a column of ash five kilometres high, prompting officials to raise the alert level.
The ash was preceded overnight by two small explosions from the crater, the Geophysics Institute said on Twitter on Friday.
The Cotopaxi volcano stands 5,897 metres high and is located 45km south of the capital Quito.
In Quito, which has a population of 2.3 million, preparations began for a million surgical masks to be handed out.
The Geophysics Institute said Cotopaxi is one of the most dangerous volcanoes in the world because of the large amount of snow at its peak and because nearby areas are densely populated.
The last time it erupted was in 1877.
The government's emergency management agency raised the alert level in areas near the volcano to yellow, which is in the middle of the scale.
The Environment Ministry declared the volcano off limits to tourists. Fifteen climbers who were preparing to scale the mountain were sent home.
Authorities are monitoring the volcano closely as they decide whether to order evacuations.
Airports in Quito and in the town of Cotopaxi remained open despite the plume of ash, the civil aviation directorate said.
In the town of Machachi, 25km from the volcano, people started wearing surgical masks.
Brian Manzano, a hotel receptionist there, said, "when I got up, I opened the door of the hotel and saw a car covered in ash. I do not know how long it has been falling."
Cotopaxi is one of eight active volcanos in Ecuador and is closely monitored by scientists.