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 2,460-metre (8,070-foot) volcano.
"Lava has flowed out of the volcano's crater already but it's just starting. It's a non-explosive eruption," Solidum told The Associated Press. "We have to verify tomorrow if it will flow continuously."
Disaster-response officials said nearly 1,000 families have been moved to emergency shelters, including some from a permanent danger zone around Mayon, since the volcano started spewing steam and ash yesterday.
A popular tourist attraction because of its near-perfect cone, Mayon lies in coconut-producing Albay province, about 340 km southeast of Manila.
The volcano has erupted about 50 times in the last 500 years, sometimes violently, endangering thousands of poor villagers who insist on living or farming in the danger zone.
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.
On May 7, 2013, the volcano suddenly spewed ash, killing five climbers, including three Germans, who had ventured near the summit despite warnings of possible danger.
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