In the early hours of Monday, a weak flow of lava began seeping out of Taal volcano – located some 70km (45 miles) south of the capital Manila.
It comes after it emitted a huge plume of ash, triggering the mass evacuation of 8,000 people from the area.
Taal is the Philippines’ second most active volcano.
Situated on an island in the middle of a lake, it is one of the world’s smallest volcanoes and has recorded at least 34 eruptions in the past 450 years.
Authorities in the surrounding province, Batangas, have declared a “state of calamity”, signifying major disruption.
On Sunday, the volcano emitted a giant plume of ash, with rumbling sounds and tremors also reported.
A total of 75 earthquakes have occurred in the Taal region, with 32 of these earthquakes ranking 2 and higher on the earthquake intensity scale, said Phivolcs.
“Taal volcano entered a period of intense unrest… that progressed into magmatic eruption at 02:49 to 04:28… this is characterised by weak lava fountaining accompanied by thunder and flashes of lightning,” the Philippine Institute of Volcanology and Seismology (Phivolcs) said in a statement.
But Phivolcs director Renato Solidum said that signs of a hazardous eruption, including “flows of ashes, rocks, gas at speeds of more than 60 kph horizontally” had not yet occurred, according to CNN Philippines.
Phivolcs has now raised the alert level from 3 to 4, out of a maximum of 5.
Authorities have also warned of a possible “volcanic tsunami”, which can be trigged by falling debris after an eruption, pushing the water and generating waves.
The Official United Nations Office for the Coordination of Humanitarian Affairs (OCHA) said more than 450,000 people are estimated to live within the 14km danger zone of the Taal volcano.
Ash fell on several areas nearby with residents advised to wear masks. One resident in metro Manila said shops had begun to run out of masks.
“When I went to my car, I saw it was covered in ash. I hurriedly went to buy a mask from a drugstore but they had run out,” Angel Bautista, a resident of Paranaque told Reuters.
The government has warned retailers not to hike mask prices amid the surging demand.
Earthquakes and volcanic activity are not uncommon in the Philippines, which lies along the Ring of Fire – a zone of major seismic activity, which has one of the world’s most active fault lines.
President Rodrigo Duterte’s office has also ordered the suspension of government work in Manila and the closure of all schools in the capital.