The 2,462-metre (8,077-foot) volcano in central Albay province - a draw for tourists because of its near-perfect conical shape - "is exhibiting magmatic eruption," the state volcanology agency said in a statement.
At "alert level 3" on a scale of 5, Mayon has increased chances of lava flows and a potential for explosive activity within weeks or even days, it added.
The agency had elevated Mayon to "alert level 2" on June 5.
It said people within a 6-kilometre (3.7-mile) radius of the volcano should be evacuated while pilots should avoid flying their aircraft close to the volcano's crater.
There are no major industries around the volcano, with most farming activities in the area consisting of subsistence crops.
Mayon is among the most active of 24 volcanoes in the country, having erupted more than 50 times in the last four centuries. The most destructive eruption came in February, 1841 when lava flows buried a town and killed 1,200 people.
The Philippines is in the Pacific "Ring of Fire," where volcanic activity and earthquakes are common.
