The white-capped volcano that looms over Mexico City emitted a terrifying low-pitched roar Friday and spewed roiling towers of ash and steam as it vented the pressure built up by a massive chamber of magma beneath its slopes. Authorities prepared evacuation routes, ambulances and shelters in the event of a bigger explosion.

Even a large eruption of the 17,886-foot (5,450-meter) cone of Popocatepetl is unlikely to do more than dump ash on one of the world's largest metropolitan areas. But the grit could play havoc with Mexico City's busy airport, and tens of thousands of people in the farming villages on its flanks could be forced to flee.

Popo, as it's commonly known, has put out small eruptions of ash almost daily since a round of eruptive activity began in 1994. A week ago, the eruptions started growing larger and authorities slightly elevated the alert level for people living nearby.

Before dawn on Friday, the mountain moved into what appeared to be a new level of activity, spitting out dozens of ash clouds and shot fragments of glowing rock down its slopes while frightening the residents of surrounding villages with deep roaring not heard in a decade.

People in the village of Xalitzintla said they were awakened by a window-rattling series of eruptions. Mexico's National Disaster Prevention Center said one string of eruptions ended in the early morning, then the volcano started up again at 5:05 a.m., with at least 12 eruptions in two hours.

"Up on the mountain, it feels incredible," said Aaron Sanchez Ocelotl, 45, who was in his turf grass fields when the eruptions happened. "It sounds like the roaring of the sea."

A 35 million cubic foot (1 million cubic meter) chamber of magma is seething about six miles (8 to 10 kilometers) beneath Popocatepetl, Roberto Quaas, director of the disaster prevention center, said at a news conference laying out emergency preparations.

Scientists have no way of predicting whether the molten rock in the chamber will be slowly released, or erupt in a powerful explosion like one on Dec. 18, 2000, that sent up a plume of red-hot rock and forced the evacuation of thousands of people who live at the volcano's base, Quaas said.

He compared the volcano to a bottle of champagne: "You could take the cork out quickly and all the gaseous material and liquid rushes out suddenly, or it could also happen slowly."

However, he said, "we know that this lava dome, sooner or later, will be destroyed by internal pressure."

Scientists have detected fracturing about 3.5 miles (5 to 6 kilometers) down, accompanied by small earthquakes measuring about 3.4 magnitude, he said.

An iconic backdrop to Mexico City's skyline on clear days, Popocatepetl sits roughly halfway between Mexico City and the city of Puebla — meaning some 25 million people live within a 60-mile radius of the volcano, Quaas said.

"These are figures that obviously alarm and concern us," he said.

President Felipe Calderon and the governors of Mexico, Puebla and Morelos states that neighbor Popocatepetl said on a live national television broadcast that they were keeping roads open around the mountain, preparing emergency shelters and making sure residents know the latest information about a potential eruption.

"It's our obligation to stay alert, to stay on guard, so we can keep carefully following developments and have the opportunity to respond quickly, if needed, and efficiently, to whatever might happen," Calderon said

Gregorio Fuentes Casquera, the assistant mayor of Xalitzintla, a village of 2,600 people about seven miles (12 kilometers) from the summit, said the town had prepared 50 buses and was sending out its six-member police forces to alert people to be ready to evacuate.

"Everyone needs to take this seriously. This buzzing, this roaring isn't normal," he said, adding that he believed about half the populace would be willing to evacuate, while the rest would want to stay.

As the quiet of the corn fields and fruit orchards was pervaded by the volcano's spooky roaring, dozens of women lined up in Xalitzintla's main square to get free face masks and bottles of water. Health authorities were giving out 10 masks and 10 bottles of water to each family, and the surgical-style masks, intended to filter out the fine ash released by the volcano, were becoming common among the town(asterisk)s students, who are required to wear them in school. Few adults wore them.

"Right now we're not scared. When it's scary is at night, when it's putting out lava," said Nancy Agustin Inclan, 14, as she removed her mask and took a break outside the gate of the town's middle school.

Webcam images on the site of the National Disaster Prevention Center showed the plume rising from the top of the peak at dawn, though clouds obscured the volcano for people further away. The Televisa television network broadcast images of red, glowing material rising from the crater and falling on its slopes.

The ash was blowing to the northeast, in the general direction of Puebla.

The recent round of eruptions ended a period of decades in which the volcano named after a legendary Indian warrior was seen as peaceful, sleeping giant, and a tourism attraction.

Homero Aridjis, a Mexican poet who has written about Popocatepetl and the sharper-edged neighboring Iztaccihuatl, named for the warrior's mythical lover, said people ascribed deep significance to the volcano's eruptions.

He noted that the last times Popo, also known as Don Goyo, had major activity were in 1994 and 2000, both presidential election years like this one, and some believed he was angry at the choice of candidates.

For others in Mexico, the eruption and a series of recent strong earthquakes are fueling speculation about what some believe to be a Mayan prophecy that the world will end in 2012.

" Popo is a godlike figure," Aridjis said. "Like the Japanese with Mount Fuji ... a natural presence, a historical one, a figure of legend."

David Gorzo Navarro, a 45-year-old street vendor in Huejotzingo, said even if there was a major eruption, people wouldn't leave.

"The people will bring mole to Don Goyo," he said. "Every year we bring an offering and today we're going to do it even earlier."

"I'm not afraid. He's my uncle," said Oscar Olate, expressing the personal relationship many in the Mexico Valley feel toward Popocatepetl, whose name means "smoking mountain" in the indigenous Nahuatl language of the Aztecs.