The eruption sent a mushroom cloud of smoke six miles above the crater and covered the capital, 33 miles away, with soggy ash during an evening rain storm. The capital's international airport closed down overnight, with passengers getting an unexpected detour to the resort of Acapulco, as the muddy ash smeared airliners' windscreens and endangered aircraft engines. On the ground, low visibility caused at lest one highway pile- up. Mexico City residents were advised to stay indoors or wear goggles and cover their noses if they went out. Many called emergency services for explanations of the strange cloud of wet ash that had replaced the city's renowned daytime smog.
It was as though someone had emptied a giant ashtray over the city and vulcanologists said the rain had helped prevent a disaster. The state of Puebla, one of three states which the 17,890ft Popocatepetl straddles, declared a "red alert" emergency, preparing to evacuate tens of thousands of residents. Most refused, saying they feared looting of their homes, which happened during the last evacuation in 1994.
Hundreds of people clogged roads out of one town, Amecameca, but most stayed home or flocked to churches to pray. In the village of Santiago Xalizintla, many residents moved to higher ground after a volcanic mudslide, cooled by the rain, threatened lower-lying homes. Against the sound of the village church bells, a loudspeaker broadcast a priest's voice, urging residents to come in and pray that there not be the kind of full-scale eruption which could bury the village.
In Mexico City, some non-government experts criticised the authorities for playing down the danger and warned that neither the national nor state governments were prepared for a potential disaster.
Some 300,000 peple, mostly farmers, live directly under the volcano - widely known as "el Popo" for short. It formed the backdrop to Malcolm Lowry's novel Under the Volcano.
The eruption began at 6pm - evening rush hour in Mexico City - on Monday and lasted half an hour. The soggy ash caused havoc on the capital's roads, bringing visibility down to 100 yards and forcing motorists to stop constantly to wipe their windscreens.
By yesterday the situation was returning to normal. Mexico City residents went back to work. Teachers came to work early to clear ash from school playgrounds. Hospitals were busy with people who had suffered burning eyes or sore throats.