TAGAYTAY CITY - Tagaytay, an upland resort city about two hours' drive south of the Philippine capital, Manila, owes much of its prosperity to a small volcano that, while very active, has not really been a threat.

The volcano, Taal, sits on a small island inside a vast lake that partly fills a caldera formed thousands of years ago. It offers a picturesque view, especially along a long ridge in Tagaytay, luring local travellers and tourists alike and they, in turn, prop up the city's economy.

Since Sunday (Jan 12), however, Taal has forced Tagaytay to shut down.

In dramatic fashion, it spewed a column of ash kilometres into the air that spread so wide it blanketed metropolitan Manila, forced major airports to suspend operations, and closed down schools, offices and the stock market.

Tagaytay was hit especially hard. The city of over 70,000 has had no electricity and running water since Sunday evening.

Roads leading to Tagaytay are caked with a layer of ash that vehicles are stirring up, forcing drivers to turn on their headlights at mid-day.

On Monday, most of the city's restaurants and amusement parks were closed. Many guests at its hotels and lodges have checked out and are headed back to Manila.

Hordes are flocking to local malls and convenience stores to purchase provisions, as they dig in and prepare for the long haul.

"We closed down at 8am when we lost power and because of the ash fall," Mr Julius Crisostomo, 29, a staff at Vista Point, a cluster of restaurants in Tagaytay, told The Straits Times.

He said there were water tanks. "But without electricity, we can't run our water pumps."

A manager at one restaurant said the local utility company had sent out an advisory that the power cut would remain for at least a week.

Adding to jangled nerves, the city has been rocked by tremors every hour since Sunday.

"I couldn't sleep. It got especially bad early on Monday, at around 2am. My room was shaking so hard I thought the mirrors would break," said Mr Jay-Ar Barcelona, 26.

He said the ground began shaking three to four times an hour early on Sunday evening,

Ms Baby Ong, 62, said she, her husband, sister and two granddaughters had stayed out of their house since 8am on Monday because of the tremors.

"We were worried our house would collapse because the ground was always shaking," she said.

She said they planned to drive around Tagaytay on Monday, till they felt safe to return to their home. If not, they plan to head to Manila and stay there for now.

Ms Ong, who lives abroad, went back for a visit to the Philippines in October. In November, she managed to join a hiking tour that took her to the mouth of the volcano.

"The water inside the crater was already boiling even then," she said.

Seismologists at the state-run Philippine Institute of Volcanology and Seismology (Phivolcs) has been monitoring Taal as early as March last year.

Still, they were surprised at how quickly the situation escalated on Sunday.

They kept the alert level at 4 on Monday, meaning a "hazardous, explosive eruption" is imminent.

But they cannot say exactly when that will happen, or when it will all end. It can be hours or days.

Ms Mariton Bornas, chief of Phivolcs' monitoring and eruption prediction division, told a news briefing it could be as short as three days or as long as seven months.

"Hopefully, it will be short because of the damaging impact a prolonged one will have on those in affected areas," she said.

Taal has erupted more than 30 times in the past 500 years.

It last erupted in 1977. A violent, sudden eruption in 1911 lasted for just three days, but over 1,300 people were killed when they could not leave their island on time. An earlier one, in 1754, lasted for seven months.

For those in Tagaytay, waiting that long is not an option.

By mid-morning on Monday, residents and groups of police and road safety workers dispatched by the central government were already sweeping ash off a highway leading to Tagaytay and also scraping centimetres of mud.

Fire trucks, ambulances and other emergency vehicles were racing across the highway, heading for towns in need of supplies and provisions for those who have to be evacuated.

Convoys of riders from the Singaporean-helmed motorcycle ride-sharing service Angkas could also be seen ferrying boxes of relief goods.

Staff at a restaurant that had to be shuttered were also keeping themselves occupied by sweeping their parking lot, as Taal volcano continued belching plumes of ash nearby.

"It's not easy to sweep. It turns to mud when wet. When it dries up it hardens, and you can't just sweep it. You have to scrape it off the floor," said Ms Mirabel de Lara, 22.

In towns and villages nearer the volcano, the situation is even bleaker.

The National Disaster Risk Reduction and Management Council reported that over 25,000 have been evacuated to safer grounds.

Most came from Batangas province, which on Monday declared a "state of emergency". Many have had to walk through ankle-deep mud for several kilometres. Some could not yet flee.

Others are just refusing to leave.

"We have a problem. Our people are panicking due to the volcano because they want to save their livelihood, their pigs and herds of cows," Mayor Wilson Maralit of Balete town told DZMM radio.

"We're trying to stop them from returning and warning that the volcano can explode again anytime and hit them," he said.

Officials expect the number of evacuees to swell, with hundreds of thousands more being brought out of harm's way.

Mayors of towns affected or receiving evacuees are asking the government to send more food and water, as well as additional troops and police.

Mayor Norberto Segunial, of Santa Teresita town, in Batangas, said he has provisions for only 500 evacuees. But he has seen over a thousand seek shelter in his town.

"They came and had dinner, breakfast here. Now, we've run out of food and water. We have very limited resources," he told ABS-CBN News.

Large numbers of displaced villagers are worrying about their homes, farms and cattle, and the uncertain future they face.

Ms Irene de Claro, a mother of four, said her father stayed in their village in Agoncillo town in Batangas, while the rest of the family fled in panic.

"We don't know too what happened to our house because the ash was up to our knees. It was very dark and the ground was constantly shaking when we left," she told The Associated Press.

"Most likely, there's nothing for us to return to. We're back to zero."