"Mayon's summit crater is now exhibiting [a] bright crater glow that signifies the growth of a new lava dome and beginnings of lava flow towards the southern slopes", Phivolcs added, explaining that magma is already at the volcano's crater.
Phivolcs has raised Alert Level 2 over the volcano, which means the "current unrest is probably of magmatic origin, which could lead to more phreatic eruptions or eventually to hazardous magmatic eruptions".
The level of danger was etched-up a notch following concerns that the phreatic or steam-driven eruptions could lead to hazardous emissions.
The ash column reached 2,500 metres in height from the crater of Mayon Volcano in Albay province, 330 kilometres south of Manila, according to the Philippine Institute of Volcanology and Seismology.
"The public is strongly advised to be vigilant and desist from entering the 6k-radius Permanent Danger Zone to minimise risks from sudden explosions, rockfall and landslides", Phivolcs said.
Earlier, the second phreatic eruption occurred also yesterday at 8:49 a.m., which lasted approximately for five minutes.
"The volcano's abnormal behaviour began late past year and it seems due for another one", Phivolcs chief Renato Solidum said.
Mayon's most violent eruption was in 1814, when more than 1,200 people were killed and a town was buried in volcanic mud.
Mayon, known for its near-perfect cone, last erupted in 2014.
Despite this, Cedric Daep, head of the Albay Provincial Disaster Risk Reduction and Management Office, said people in evacuation centres may be allowed to return home later today, unless Phivolcs raised the volcano alert level further.
Residents were evacuated from two villages near the volcano on Saturday.
Residents have been urged to protect against inhaling the ash by wearing masks or covering their noses and mouths with damp cloths.
It also noted that a "faint crater glow" was observed on Saturday evening and Mayon has been "slightly swelling" since October and November past year.