Volcanologists say a major eruption could take place at the Mayon volcano in just days or weeks. The Philippines' most active volcano spewed burning mud and rocks on Tuesday, prompting authorities to start evacuating about 50,000 people from nearby cities and towns.

[Bernardo Alejandro, Regional Director, Office of Civil Defense]:
"We hope to execute the evacuation at the soonest time, because we do not have to wait for Mayon volcano to erupt to start evacuating people. So we're here to aim for what we call a 'zero casualty.'"

On Monday, the alert level around Mayon had been raised to number 3, indicating volcanic materials are now near the top of the crater.

Level 4 means an eruption is imminent and Level 5 means an eruption is in progress.

Disaster workers have distributed masks to prevent respiratory illnesses.

The air has become hot and irritable in the area, and smells of rotten eggs.

The fleeing residents are temporarily being housed in school buildings and public gymnasiums, where local government officials set up food and relief centers.

Humanitarian and non-government groups have pledged to send food, water, medicine and blankets.

The Mayon volcano is famed for its near-perfect cone shape and attracts numerous tourists.

Mayon is the most active of 22 volcanoes in the country, having erupted more than 50 times in the last four centuries. The last time Mayon erupted was in 2006.

The Philippines lies on the "Ring of Fire," a belt of volcanoes circling the Pacific Ocean that is very prone to earthquakes.