The Philippines' most active volcano has belched out huge lava fragments that rolled about a kilometer (half a mile) down its slope, prompting authorities to evacuate thousands of villagers, officials said Tuesday.

The Philippine Institute of Volcanology and Seismology raised the alert level to "critical" for Mount Mayon in eastern Albay province late Monday, meaning an explosive eruption is possible within weeks. The level was raised after an escalation of restiveness was recorded overnight, including the ejection of glowing rocks from the summit and dozens of low-frequency volcanic earthquakes.

Molten lava has accumulated at the top of the 2,460-meter-high (8,070-foot-high) volcano's crater, creating a glow in the clear night sky Monday that sparked both awe and fear among spectators and villagers.

"Thousands have been evacuated since last night," Cedric Daep, a safety officer in Albay, said by phone.

Volcanologist Ed Laguerta said he saw huge glowing lava fragments and super-hot boulders rolling down from Mayon's crater for the first time late Monday, when thick clouds parted and revealed the volcano's summit. He said the fragments could be seen from as far as 12 kilometers (7 miles) away.

Mount Mayon, a popular tourist site known for its near-perfect cone, lies in coconut-producing Albay province, about 340 kilometers (210 miles) southeast of Manila. On some clear nights, tourists can see the lava glow from the safety of their hotels in Albay's capital, Legazpi city.

Albay Gov. Joey Salceda ordered the evacuation of up to 12,000 residents within an 8-kilometer (5-mile) radius of the volcano's crater. Villagers within 6 kilometers (4 miles) of the volcano, which the government has declared a permanent no-go danger zone, face mandatory evacuation.

Disaster-response officials convened an emergency meeting Tuesday to map out steps to avert casualties.

Mayor Herbie Aguas said his farming town of Santo Domingo, among the closest to the volcano, has a frightening legacy from Mayon. The volcano nearly wiped out the municipality's entire population in an 1897 eruption with its pyroclastic flows — superheated gas and volcanic debris that race down the slopes at high speeds, vaporizing everything in their path.

"We are praying that it would not be the worst-case scenario," Aguas said, adding that nearly 4,000 of the 40,000 residents in his town who live within a government-declared danger zone had started to evacuate to safer areas.

The volcano has erupted 50 times in the last 500 years, sometimes violently, endangering thousands of poor villagers who insist on living or farming in the government-declared permanent danger zone.

Villagers living near the volcano 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 ventured near the summit despite warnings of possible danger.

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Associated Press writers Oliver Teves and Jim Gomez contributed to this report.