Searing gas and debris raced down the slope of a Philippine volcano as rumbling Mayon entered its most dangerous phase after weeks of ejecting lava and ash, a scientist said Sunday.

The burst of pyroclastic flow on Saturday marked the first time Mayon shot out fast-moving hot gas and rock fragments after weeks of showing signs of a major eruption, volcanologist Ed Laguerta said.

"This is really the dangerous phase," said Laguerta. "We're not worried much with lava flows because they're slow moving, but pyroclastic flows travel at such high velocity and could destroy almost everything in its path."

CountryWatch: Philippines

Blistering gas and volcanic debris can travel at least 37 miles per hour, he said.

The Philippine Institute of Volcanology and Seismology predicted more pyroclastic flows in the next few days because of Mayon's continuing unrest and urged villagers not to venture into an extended danger zone around the volcano.

Government volcanologists have been saying 8,077-foot Mayon may be gearing up for an explosive eruption.

Authorities have ordered the mandatory evacuation of people within the volcano's danger zone, and nearly 45,000 people have been moved to 27 temporary shelters, mostly school buildings in Albay province, southeast of Manila.

President Gloria Macapagal Arroyo on Saturday visited villagers in shelters. She ordered officials to speed up efforts to improve conditions at overcrowded evacuation centers and ensure adequate stocks of food and medicine.

Mayon continued to show signs of restiveness on Sunday, emitting abnormally high levels of sulfuric dioxide and puffing ash at least six times. Volcanologists also say they continue to detect a high number of volcanic earthquakes, mostly unfelt by humans.

Mayon, a popular tourist attraction because of its nearly perfect conical shape, is one of the Philippines' 22 active volcanos.

Its most violent eruption in 1814 killed more than 1,200 people, many of whom sought shelter in a church that was buried by lava in Cagsawa town.

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