State volcanologists raised the alert level on the cone-shaped, 8,070-foot Mayon volcano overnight to two steps below a major eruption after ash explosions and dark orange lava fragments glowing in the dark trickled down the mountain slope.
Nearly 50,000 people live in a five-mile radius around the mountain, and authorities began moving thousands of them in case it erupts, Albay provincial Gov. Joey Salceda said.
More than 20,000 people were evacuated to safety by nightfall Tuesday, Salceda said, adding he has placed central Albay province, where Mayon is located, under a "state of imminent disaster," which will make it easier for him to draw and use emergency funds.
"Whatever the volcano does, our target is zero casualty," Salceda told The Associated Press.
Albay province lies about 210 miles southeast of Manila.
Salceda said he had decided to cancel a trip to Copenhagen, where he was to attend the U.N. climate conference to discuss his province's experience with typhoons and other natural disasters.
He said he would appeal for foreign aid to deal with the expected influx of displaced villagers to emergency shelters.
The first of 20 vehicles, including army trucks, were sent to villages to take residents to schools and other temporary housing, provincial emergency management official Jukes Nunez said.
"It's 10 days before Christmas. Most likely people will be in evacuation centres, and if Mayon's activity won't ease down we will not allow them to return to their homes," Nunez said. "It's difficult and sad, especially for children."
Although the alarm has been sounded, life throbbed normally in many laid-back farming villages near the restive volcano. Throngs of farmers flocked to the town hall in Guinobatan, which lies near the danger zone, for a Christmas party, then headed home bearing gifts.
Village leader Romeo Opiana said the 249 residents in his farming community of Maninila, near the volcano, readied packs of clothes but no one had left. An army truck was parked nearby, ready to haul people if the threat grows.
"We're ready, but we're not really alarmed," said Opiana, 66. He could not remember how many times he had seen Mayon's eruptions since childhood.
Magma had been rising at the volcano over the past two weeks and began to ooze out of its crater Monday night, but it could get worse in coming days, said Renato Solidum, head of the Philippine Institute of Volcanology and Seismology.
"It's already erupting," Solidum told the AP. He said the volcano had so far only gently coughed out red-hot lava, which had flowed half a mile (half a kilometre) down from the crater.
Some classes were suspended indefinitely near the danger zone. Officials will find a way to squeeze in classes in school buildings to be used as shelters, Salceda said.
Residents in Albay are used to moving away from Mayon, which spewed ash last month and prompted the evacuation of some villages.
About 30,000 people were moved when it last erupted in 2006. Typhoon-triggered mudslides near the mountain later that year buried entire villages, killing more than 1,000 people.
Mayon's most violent eruption, in 1814, killed more than 1,200 people and buried a town in mud. A 1993 eruption killed 79 people.
The Philippines lies along the Pacific "Ring of Fire," where volcanic activity and earthquakes are common. About 22 out of 37 volcanos in the archipelago are active.