Mayon, a volcano famed in equal parts for its near-perfect cone and brutal volatility, had begun to stir again with magma rising to the top and small earthquakes rattling deep inside, authorities said.
"A hazardous eruption is possible within weeks," the director of the state volcanology agency, Renato Solidum, told AFP.
Residents within an eight-kilometre (five-mile) radius will be forcibly evacuated, said regional civil defence director Bernardo Alejandro.
He said authorities expect to evacuate almost all of the 50,000 people in the danger zone -- a picturesque coconut farming area near the Pacific coast -- within three days.
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"They don't want to leave their houses and their livelihood.... These are coconut and orchid farmers with chickens, pigs and carabaos (water buffalo)."
Locals involved in the tourist industry were expecting Mayon's latest burst to be a mini-boom.
The area, about 330 kilometres (200 miles) southeast of Manila, is already a draw for visitors who want to see Mayon's cone, sample the region's spicy cuisine and visit its beautiful beaches.
"It's dramatic, like a fireworks show.... When there's nothing happening (in the volcano) it's all dark around here, but now it's picture-perfect," Calleja told AFP.
Calleja said that when Mayon became active in the past, his clients often requested night tours to see the glowing crater.
Aljon Banares, who works for a backpackers' inn 12 km (7.5 miles) from the volcano, was also preparing for more visitors.
Four foreign tourists and their local tour guide were killed when Mayon last erupted, in May 2013.