MEXICO CITY (Reuters) - Popocatepetl volcano exhaled a plume of ash on Saturday morning and layered Mexico City in white volcanic dust, sending surprised residents seeking cover and coating cars and buildings.

"It was like rain. Our eyes were burning," said Isabel Hernandez, who was caught outside with her two children when the ash fell on the city. All three covered their mouths with wet pieces of cloth.

Popocatepetl, known as Popo, 40 miles southeast of the capital, sent up a 1.8-mile high plume of ash and incandescent fragments at 9:20 a.m. local time.

Hours later, winds carried the cloud of ash to Mexico City, turning the sky dark and raining down fine dust.

Residents rushed into pharmacies to buy bright blue surgical masks to cover their mouths, while radio programs warned people to keep their heads covered and wash the ash off their cars carefully, as the fine volcanic particles can scratch paint.

The dust was especially thick in the south of the city, with radio reporting low visibility in those neighborhoods.

"Everything got dark here, as if it were raining. Some people were scared," said Alfredo Rosales, who was in the southern part of the city when the ash fell.

"In the south you can't even see anything," said taxi driver Rene Pena, 31.

Authorities said the ash was not a health threat.

Disaster prevention officials at Cenapred said the amount of ash that fell on the city was minimal and the volcano was stable.

"The volcano is very calm. The winds brought the ash toward the city but that's something we expect," an official said.

Mexico's international airport reported little ash had fallen there and remained open, with one flight delayed for 15 minutes and another landing at a different airport.