The sky over the capital was virtually blacked out, and officials were quoted as saying they expected "several tonnes of ash" to fall in Quito.

A huge cloud quickly appeared over Quito

The first activity came at 0712am (1212GMT) and work stopped as people ran to watch a spectacular mushroom-shaped cloud forming high in the sky.

Eight minutes later there was another strong explosion, followed by a third, less intense, at 0807am (1307GMT).

Schools are closed and many of Quito's 1.5 million inhabitants have taken to wearing face masks.

Yellow alert

The city's international airport is closed again after a brief re-opening on Thursday morning.

People are advised to wear facemasks

The capital is under "yellow alert", which means the authorities assume Guagua Pichincha could erupt in weeks or months but probably no sooner.

The volcano began smoking on Friday but officials lifted a heightened state of alert across the city on Monday after signs that the activity had stabilised.

"The volcano has not changed its activity," said Ecuadorian Institute of Geophysics director Hugo Yepez on Monday.

"This [ash fall] is just beginning and one has to get used to it."

Months or years

Hundreds of children live in shelters

The volcano could continue spewing increasing amounts of ash and vapour for many more months or even years.

Volcanologists say the activity is part of a slow decompression process and there is no danger of the volcano exploding.

Quito, which is 2,900 metres (9,600ft) above sea-level in the Andes, is surrounded by nine active volcanoes. Guagua Pichincha is about 12km (7 miles) away.

Pichincha has erupted at least 25 times, the last recorded activity being in 1993.