Experts said the Mayon volcano could produce a massive eruption at any moment. ((Bullit Marquez/Associated Press))

Nearly all of the 47,000 people living on the slopes of a volcano in the central Philippines have fled to emergency shelters as officials prepared for a possible massive eruption Tuesday.

Experts said the 2,460-metre Mayon volcano in coastal Albay province could produce a dangerous eruption at any moment, though it's difficult to predict.

"Hazardous eruption ... can happen today or in the next few days," said chief state volcanologist Renato Solidum. "Nobody has a 100 per cent accuracy."

Ash columns were seen rising during a cloud break and Solidum said his team recorded 1,266 volcanic quakes in the last 24 hours, down from nearly 2,000 the previous day. He said that while the quakes were fewer, they were larger.

The emission of sulfur dioxide — an indication of magma rising inside the volcano — was measured at nearly 6,000 tonnes per day, slightly less than Sunday's output, but still very high, Solidum said. The normal gas emission is around 450 tonnes per day.

"Technically, Mayon volcano is already erupting because lava has oozed out," said Mahar Lagmay, professor of geological studies at the University of the Philippines. A bigger, explosive eruption is still possible but hard to predict, he said.

In the past week, tens of thousands of residents living within eight kilometres around Mayon have been transported in army trucks to school buildings and gymnasiums in and around the provincial capital of Legazpi, where they were given sleeping mats and food supplies to last them through Christmas holidays, said Jukes Nunez, a disaster management official.

Scientists have raised Mayon's alert level to one step below a hazardous eruption. The only higher level is when a major eruption is in progress.

Pyroclastic flows feared

Experts fear a major eruption could trigger pyroclastic flows — superheated gas and volcanic debris that race down the slopes at high speeds, incinerating or vaporizing everything in their path.

More extensive explosions of ash could drift toward nearby towns and cities, including Legazpi, about 15 kilometres away.

In Mayon's other eruptions in recent years, pyroclastic flows had reached up to six kilometres from the crater.

"The probability of survival in an eruption is zero if you're in the danger area. The solution is obviously distance," Albay Gov. Joey Salceda said.

Mayon has erupted nearly 40 times over 400 years. About 30,000 people were moved during the last eruption in 2006. An eruption in 1993 killed 79 people.

The most destructive came in 1814, killing more than 1,200 people and burying a town in volcanic mud. The ruins of the church in Cagsawa have become a tourist attraction.

In 1991, Mount Pinatubo exploded in the northern Philippines in one of the biggest volcanic eruptions of the 20th century, killing about 800 people.