The city was reported to be calm but nervous as a thin blanket of ash settled over several neighbourhoods.

The airport and schools were closed and choking residents were forced to wear face masks.

Guagua Pichincha produced a huge cloud of ash and gas on Tuesday

The skies over Quito darkened when the cloud of ash, 11km (7 miles) high and 44km (28 miles) across, appeared on the city's western horizon at about 1500 local time (1800 GMT) on Tuesday.

City authorities said the renewed activity did not mean a major eruption was imminent.

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 Ecuadorean Institute of Geophysics director Hugo Yepez. "This [ash fall] is just beginning and one has to get used to it."

The city's international airport was closed to prevent ash damaging planes and school has been suspended until further notice.

Months or years

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's 1.5m inhabitants have been urged to buy face masks and eye goggles, and to ensure they have emergency supplies.

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

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