The plume coming from the Taal volcano, south of the capital Manila, stretched about 1km (0.6 miles) into the sky.
Rumbling sounds and tremors were also reported around Taal - the Philippines' second-most active volcano - on Sunday.
Authorities raised the alert level, warning that a "hazardous eruption" was possible within weeks.
The Philippine Institute of Volcanology and Seismology also warned residents living nearby of a possible "volcanic tsunami", while flights were suspended at Manila's international airport because of the volcanic ash.
Ash began falling in nearby areas on Sunday, with residents advised to wear masks.
A man in the city of Tagaytay, a popular holiday destination, told Reuters news agency he was having lunch when he heard a rumbling sound.
"We saw the volcano erupting. It rained and some small pebbles fell to the ground," Jon Patrick Yen said. "I did not expect to see such spectacle."
Lucy McFadden, a Filipina based in the UK, plans tours for the company Philippine Trails. She said eight tourists and two guides, including her brother, were on a boat returning from the volcano when it erupted.
"About 30 minutes into their journey, the volcano erupted without warning. My brother was so scared because he knew he had a narrow escape," she told the BBC.
The Philippine institute has urged people living around the volcano, located on an island in the middle of Taal lake, and two nearby "high-risk" municipalities to evacuate.
Taal is one of the world's smallest volcanoes. It has recorded at least 34 eruptions in the past 450 years.