Thousands of people in the Philippines have fled their homes after the country's most active volcano oozed lava and shot up plumes of ash on Tuesday. State volcanologists raised the alert level on the cone-shaped, 2,460-metre Mayon volcano to two steps below a major eruption, saying the activity could get worse in the coming days.

"It's already erupting," said Renato Solidum, head of the Philippine Institute of Volcanology and Seismology.

More than 20,000 people living near the volcano were evacuated to safety by nightfall Tuesday, said Gov. Joey Salceda of Albay province, where Mayon is located about 340 kilometres southeast of Manila.

Nearly 50,000 people live within an eight-kilometre radius of the volcano.

Salceda said Tuesday that the province of Albay has been placed under a "state of imminent disaster" to allow for better mobilization of resources to handle the evacuation.

Emergency management officials said residents were sent to schools and other temporary housing, where they will likely remain over the Christmas holidays if the activity does not die down.

"It's difficult and sad, especially for children," said provincial official Jukes Nunez.

Mayon has erupted about 50 times over the last few hundred years, making it the most active volcano in the country. An eruption in 1814 killed 1,200 people.

More recently, the volcano spewed ash and lava from July to October 2006, sending about 30,000 people fleeing from their homes. In December 2006, typhoon-triggered mudslides along Mayon's slopes buried entire villages, killing more than 1,000 people.