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TAGAYTAY CITY -- (UPDATE) Gray ash from Taal Volcano blanketed large parts of Southern Luzon and Metro Manila on Monday, shutting the country's main airport overnight, prompting a health advisory, and forcing thousands to flee as authorities kept vigil for a possible hazardous eruption.

The Ninoy Aquino International Airport reopened for departures at 10 a.m. and arrivals at 12 p.m., aviation authorities said as they managed a throng of passengers at the terminal.

Relief workers trudged through ash-covered roads with their headlights on at dawn in Agoncillo, Batangas. In the capital, people lined up for face masks, some at 8 times the retail price as they cleaned ash-covered cars and roofs.

Taal spewed lava on Monday, a day after it blew ash and steam into the air. Alert Level 4, the second-highest in a 5-step scale, was raised, meaning a "hazardous" eruption was possible in days.

"Napakabilis ng pagbabago ng kanyang kundisyon (The condition is changing very rapidly)," said Phivolcs Director Renato Solidum.

In a "worst case scenario," Solidum told DZMM that magma could come out of the volcano as it shoots ash and rocks into the air.

• 'A rapid escalation': Phivolcs chief warns rising magma could cause more volcanic activity
• Alert level 4 raised over Taal, volcanic tsunami possible: Phivolcs

Located in the middle of a picturesque lake that straddles the tourist city of Tagaytay and a heritage town of the same name, Taal's last eruption was in 1977, Solidum earlier said.

Inhaling ash could lead to cough and difficulty and breathing. It could also irritate the eyes, the Department of Health said. The public was also warned about the possibility of roofs caving in due to accumulated ash.

• LIST: Canceled flights due to Taal ash eruption, Monday Jan. 13
• Muddy roads hamper Batangas town evacuation from Taal wrath

ASH GROVE

In Tagaytay City, a popular picnic grove was covered in ash, leaving some tourists in a bind on how to leave. The city's hospital asked patients to transfer to areas that are far from the volcano.

"Nalulugmok ako kasi hindi namin alam saan dadaan... nahirapan kami papunta dito kasi madulas yung mga paakyat dahil sa putik," said Herbert Sidocon, a baker from Imus who was watching over family at the hospital.

Nipa huts were covered in grime, while tree branches fell under the weight of the ash.

“Naka-ilang bagyo na dito pero hindi bumabagsak 'yang mga puno kahit gano kalakas [ang bagyo]. Yang mabigat na putik lang ang nakapagpabagsak diyan,” said Nila Asito, who has been working as a maintenance staff in the Picnic Grove for 16 years.

(I've been through strong typhoons, the trees withstood them. Only the heavy mud brought the down.)

UNTIL FURTHER NOTICE

The Ninoy Aquino International Airport was shut until further notice as authorities assess the effect of the ash cloud. Volcanic ash in the air is dangerous since it could not be easily detected by radar, authorities said.

Some Manila-bound flights could be diverted to Clark Airport, father north from the volcano, authorities said.

Earlier, Phivolcs cited a threat to aircraft from "ballistic fragments." Transport secretary Arthur Tugade had instructed aviation officials to "do whatever is necessary in the interest of public safety," said a joint statement from air and transport authorities.

School and government offices in affected areas were shut.

Several tremors were felt within the vicinity of the volcano, seismologists said.

More than 2,000 residents living on the volcanic island, which lies inside a bigger lake formed by previous volcanic activity.

Solidum told Agence France-Presse officials would also order the evacuation of people living on another island nearby if the situation worsens.

Apart from the ash, some particles up to 6.4 centimeters (2.5 inches) in diameter, larger than a golf ball, had reportedly fallen in areas around the lake, Phivolcs said early Monday. By that time 52 volcanic earthquakes had occurred, it said.

Earthquakes and volcanic activity are not uncommon in the Philippines due to its position on the Pacific "Ring of Fire", where tectonic plates collide deep below the Earth's surface.

Two years ago, Mount Mayon displaced tens of thousands of people after spewing millions of tonnes of ash, rocks and lava in the central Bicol region.

The most powerful explosion in recent years was the 1991 eruption of Mount Pinatubo, about 100 kilometers northwest of Manila, which killed more than 800 people.

It spewed out an ash cloud that traveled thousands of kilometers in a matter of days and was blamed for damaging nearly two dozen aircraft.

-- with a report from Agence France-Presse