Mayon, renowned for its nearly perfect cone shape, produced five small explosions around daybreak on Wednesday, with the most powerful shooting ash more than 1,500 feet in the sky.
It then began to produce a continuous stream of lava down its flanks, 300 miles south of the capital, Manila.
Vulcanologists predict that the mountain could erupt for months or punctuate its current active phase with a violent explosion.
In 2006, Mayon emitted ash and oozed lava for two months, depositing a large among of ash and other debris on its slopes.
Subsequent heavy rainfall from passing Typhoon Durain triggered a huge slide of the volcanic material that killed more than 1,000 people below.
Mayon has produced 49 documented eruptions since records began in 1616.