Renato Solidum, head of the Philippine Institute of Volcanology and Seismology, said his agency is on alert for a possible hazardous eruption of the Mayon Volcano in Albay province, 360 kilometres south of Manila.
The institute said it recorded 197 earthquakes and 15 ash explosions overnight.
"These explosions produced dirty white to grayish ash columns that reached heights from 100 to 2,000 metres above the summit," it said in a bulletin. "The advancing lava flow has reached approximately 4 kilometres downslope from the crater."
"The possibility of hazardous volcanic eruption is high," the agency warned as it called on people leaving within an 8-kilometre radius to leave their homes.
Relief officials said more than 40,000 people have already been moved to 25 evacuation centres in eight municipalities around Mayon, famous for its almost perfect cone.
The 2,472-metre volcano has erupted about 50 times since 1616. It last erupted in July 2006, forcing more than 30,000 people to flee their homes.
Mayon's most violent eruption was in 1814 when more than 1,200 people were killed and a town was buried in volcanic mud. An eruption in 1993 killed 79 people. dpa jg ls