At least 12,800 people have been evacuated from the area around the mountain as volcanic activity intensifies.
Lava began to appear on Mayon’s slopes on Sunday night and Teresito Bacolcol, director of the Philippine Institute of Volcanology and Seismology said that authorities were on alert for further developments.
“What we are seeing now is an effusive eruption,” Bacolcol told The Associated Press news agency. “We are looking at this on a day-to-day basis.”
At least 12,800 people living within a 6km (3.7 mile) radius of the Mayon volcano have been evacuated since last week. Bacolcol said there was a possibility the high-risk zone could be expanded if the eruption turn violent.
The area below Mayon, about 330km (205 miles) southeast of the capital Manila, is designated a permanent danger zone but many thousands of people continue to live there because they have nowhere else to go.
Volcanologists say the lava is a further sign of increased activity, noting that there were also about 260 rockfall events in the past 24 hours, compared with 177 in the previous 24 hours. More tremors were also recorded – 21 in the last 24 hours compared with two previously.
Sulphur dioxide emissions had also tripled on Saturday, according to state volcanologists.
“There is a concomitant health risk while being close to the eruption because of inhaling sulphur dioxide gas or the particulate matter of ash-falls,” Health Secretary Teodoro Herbosa told a press briefing on Sunday.
The alert level for the volcano remains at three on the Philippines’s five-step system, meaning that there is a tendency towards a hazardous eruption in weeks or days.
The province of Albay, where the conical Mayon is a popular tourist attraction, was placed under a state of emergency on Friday to allow for quicker distribution of any disaster relief funds in the event of a major eruption.
Mayon is one of 24 active volcanoes in the Philippines and last erupted violently in 2018, displacing tens of thousands of villagers.
The archipelago is located on the so-called Pacific “Ring of Fire”, the rim of seismic faults where most of the world’s earthquakes and volcanic eruptions occur. It also sees frequent typhoons and tropical storms with a typhoon forecast to hit the area later this week.
The country’s most powerful eruption in recent decades was Mount Pinatubo in 1991, which killed more than 800 people and produced an ash cloud that travelled thousands of kilometres.