First light brought clear the extent of the overnight explosive eruption at Mount Kelud on Java Island, though there was no immediate word on any casualties.
Booms from the mountain could be heard 130km away in Surabaya, the country's second-largest city, and even further afield in Jogyakarta.
Ash covered the ground in both cities and was still falling, according to witnesses and accounts on social media. TV footage from towns closer to the peak showed farmyard animals covered in ash.
Transport ministry spokesman Bambang Ervan said Jogyakarta, Solo and Surabaya airports were closed due to reduced visibility and the dangers posed to aircraft engines by ash.
The 1,731m Kelud had been rumbling for several weeks.
Muhammad Hendrasto, head of the country's volcano monitoring agency, said the mountain in East Java province erupted violently about 90 minutes after authorities raised its status to the highest level.
Officials late Thursday urged about 200,000 people living in 36 villages within 10km of the crater to evacuate. It was unclear how many people heeded that warning, though thousands fled when the blasts began.
Kelud is among about 130 active volcanoes in Indonesia. The archipelagic nation is prone to volcanic eruptions and earthquakes because of its location on the so-called "Ring of Fire" - a series of fault lines stretching from the Western Hemisphere through Japan and Southeast Asia.
Due to its population density and fertile soils that volcanic slopes provide, hundreds of thousands of Indonesians live close to active volcanos. They have learnt to live with the rumblings, and frequently ignore orders to leave.
The last major eruption at Kelud was in 1990, when it kicked out searing fumes and lava that killed more than 30 people and injured hundreds. In 1919, a powerful explosion that reportedly could be heard hundreds of miles away killed at least 5,160.
Earlier this month, Mount Sinabung in North Sumatra province erupted as authorities were allowing thousands of villagers who had been evacuated to return to its slopes, killing 16 people. Sinabung has been erupting for four months, forcing the evacuation of more than 30,000 people.