Tens of thousands of villagers in the Philippines fled their homes on Tuesday as one of the nation's most active volcanoes spewed lava and sent ash plumes high into the sky, authorities said.

Soldiers and police marshalled the evacuation from the so-called "danger zone" around the foothills of Mayon volcano, amid concerns a big eruption could occur at any moment.

"After the series of ash puffs and ash explosions of 1,000 metres (high), we cannot rule out a major explosion," Cedric Daep, the head of the disaster relief operations in the eastern Bicol region, told reporters.

Daep said the authorities aimed to evacuate nearly 50,000 people from villages within eight kilometres (five miles) of the volcano by Thursday.

The Philippine Institute of Volcanology and Seismology raised the alert level for Mayon to three on Monday after lava was seen dripping from the crater, and evacuations began immediately.

Level three means an eruption is likely in the very near future. The scale runs from one to five, with five meaning an eruption is occurring.

However the volcanology institute also said Mayon could yet calm down without an eruption.

Mayon, which sits above a farming area about 330 kilometres (200 miles) southeast of Manila, has erupted 48 times since records began, claiming thousands of lives.

After the most recent eruption in August 2006, huge deposits of volcanic ash were left on its slopes. When typhoon Durian hit the same area in December of that year, it caused a landslide of volcanic ash that killed over 1,000 people.

In 1814, more than 1,200 people were killed as the lava buried the town of Cagsawa.

However the 2,460-metre (8,070-feet) volcano remains a popular tourist attraction, and is famous for its perfect cone.

The Philippines is part of the so-called Pacific "Ring of Fire" that is known for its volcanic activity. The Philippine volcanology institute lists 22 active volcanoes in the country.

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