Mayon last erupted in 2014, with lava flows forcing thousands of people to evacuate.
The cone of Mount Mayon is nearly completely obscured by ash and steam.
Phivolcs recorded a third phreatic eruption at 11:43 a.m. yesterday.
Renato Solidum, who heads the volcanology institute, said three steam-explosions at the volcano since Saturday have spewed ash into nearby villages and may have breached solidified lava at the crater and caused lava to start to flow out of the 8,070-foot volcano.
Mayon volcano in Albay province spewed a smoke cloud on Sunday for the third time in less than 24 hours, state seismological office Phivolcs said as hundreds of residents fled to safer ground.
Yesterday's eruption unleashed ash, rocks and a strong smell of sulphur.
Mayon's most destructive eruption was in February 1841, when lava flows buried a town and killed 1,200 people.
And just a few hours later, the threat was raised again from two to three as Phivolcs warned of "hazardous eruption" within weeks or days as Mayon Volcano has exhibited "high unrest" and its magma is at the crater.
Villagers living near the volcano, which last erupted four years ago, have erected huge white crosses at the entrance of their neighborhoods, hoping they will protect them from harm.
Phivolcs chief Renato Solidum said the volcano appeared due for another eruption as it has been displaying abnormal behaviour since late previous year.
Mayon, one of the most active volcanoes in the country, first spewed ashes, which reached 2.5 kilometers (1.5 miles) in a phreatic eruption, lasting for nearly two hours on Saturday afternoon.
Since Saturday's first eruption, Phivolcs said it had recorded 158 rockfall events and urged people to stay away from a 6km radius Permanent Danger Zone and a 7km Expanded Danger Zone on the volcano's southern flank.
Those within the slope of the volcano, but outside the 6-km danger zone, were told to take precautionary measures against possible roof collapses due to accumulated ash and rainwater.
"We have not reached the critical level", he said in a radio interview.
"Alert level 3 is what we considered critical, 4 is when eruption is imminent, and 5 is eruption in progress", he said, adding that people in evacuation centers may be allowed to return homes if Phivolcs does not raise the level.