The volcano is normally a major tourist destination and has had relatively mild eruptions since January. But on Saturday, a pyroclastic flow, a volcanic avalanche of hot rocks, ash and gas, suddenly erupted and poured down onto populated areas.
"Some villagers panicked, but we advised them to stay calm," said Caloy Baldo, mayor of the town of Camalig.
No injuries have been reported, but the dangers are evident from the fact that four water buffaloes have been killed by ash fallout.
"The ash rain was so thick we had no visibility at all, even on the national highway," Baldo said.
The alert level has now been raised in over 100 villages around the 2,500-meter-high volcano.
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