Up to 50,000 people are expected to be moved out of the region as the Philippine Institute of Volcanology and Seismology said an explosive eruption appeared imminent.

There were six successive volcanic explosions in 40 minutes early on Monday prompting officials to raise the alert level to four - level five represents an ongoing eruption.

The danger zone on the southern side of the volcano has been extended to 8km.

Experts have advised local officials in the central province of Albay to order the evacuation of 28 areas around the mountain, including parts of Legaspi city and the towns of Camalig, Daraga, Ligao, Malilipot, Santo Domingo and Tabaco.

Lava flows

Mayon began abnormal activity in February and lava began flowing down the slopes on July 14.

Volcanic explosions sent columns
of ash 800m into the air

Army trucks and other government vehicles have been sent to move residents to about 30 evacuation centres outside the danger zone – mainly in schools and government buildings.

The centres were set up after the activity in July - water and electricity supplies were connected and food stockpiled.

Last week, government troops were sent to the area to keep sightseers away from the edge of the advancing lava flow.

Volcanic earthquakes

Earlier, volcanologists said they detected 21 low-frequency volcanic earthquakes in the past 24 hours. Renato Solidum Jr, head of the volcanology institute, said the lava flow could continue, or the eruption could shift from quiet to explosive.

Mayon has had 47 eruptions in recorded history, the latest was in June 2001. In the 19th century the volcano buried the town of Cagsawa killing an estimated 1,000 people.

Mayon is one of 22 active volcanos in the Philippines.