The force of the eruption - the strongest so far - could be felt five miles (eight kilometers) away.
"This was a big one," said 37-year-old Anto Sembiring, still shaken after abandoning his coffee shop in the middle of the danger zone. "We all ran as fast as we could. ... Everyone was panicking."
The eruption of Mount Sinabung on Sunday and Monday - which caught many scientists off guard - forced more than 30,000 people living along its fertile slopes to evacuate to cramped emergency shelters in nearby towns.
Wearing surgical masks to protect themselves from the smoky air, many have complained about the steadily deteriorating conditions, from poor sanitation and short food supplies to having to sleep on hard, cold floors.
"It's especially bad for the kids," said Yacoubus Runtuwene, a World Vision relief worker. "We're staring to see a lot of respiratory problems, diarrhea and eye irritations."
Tired of waiting, thousands of people started returning to the mountainside earlier this week so they could clean up their soot-covered homes and salvage what they could from their vegetable farms and rice paddies.
But several hours before Friday's pre-dawn blast, a new alert was issued.
Some people trudged back down the slopes, carrying blankets, clothes and food. A handful of others insisted on staying, even after the new explosion, which caused the entire mountain to violently tremble for five minutes.
"We're not going anywhere," said Razia Barimbing, who was among 50 men refusing to budge, saying they had to protect abandoned villages a few miles (kilometers) from the crater's mouth against looters.
"It's so sad to see this," said the 35-year-old farmer, pointing to the white dust blanketing houses, gardens and even livestock. "We just want this to be over, so we can pull our lives back together, and get our children back in school."
The air was thick with the smell of sulfur and, despite a soft drizzle, heavy smoke limited visibility to just a few yards (meters). Some small domestic hopper flights had to be diverted, according to Bambang Ervan, the transportation ministry's spokesman. International air travel was unaffected.
Mount Sinabung had last erupted in 1600, and government vulcanologists acknowledged they had made no efforts before the mountain started rumbling last week to sample gases or look for rising magma or other signs of seismic activity.
They were too busy with more than 129 active volcanoes in Indonesia, a seismically charged region 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.
They said from now on they will be watching it very closely.
"It's still going off, even now," said Surono, who heads the nation's volcano alert center, noting there was a rather strong burst Friday evening. "You can't see it because of the heavy fog around the crater."
There are fears that current activity could foreshadow a much more destructive explosion in a few weeks or months, though it is possible, too, that the mountain will go back to sleep after letting off steam.
The archipelagic nation has recorded some of the largest eruptions in history.
The 1815 explosion of Mount Tambora buried the inhabitants of Sumbawa Island under searing ash, gas and rock, killing an estimated 88,000 people.
The 1883 eruption of Krakatoa could be heard 2,000 miles (3,200 kilometers) away and blackened skies region-wide for months. At least 36,000 people were killed in the blast and the tsunami that followed.
Associated Press Writer Niniek Karmini contributed to this report from Jakarta.