One woman died of a heart attack as she fled, but no other casualties were reported.
Mount Lokon, located in northern Sulawesi province, unleashed its first powerful eruption at 10:46 pm (local time) yesterday, said Brian Rulrone, a disaster management agency official.
That blast was followed by a second just after midnight and a third at 1:10 am (local time) today.
Glowing lava cascaded from the mouth of the crater, triggering forest fires along its western slope, said Ferry Rusmawan, an official at the nearest monitoring post, adding that activity remained high and another eruption appeared imminent.
The 5,741-foot (1,750-meter) mountain continued to rumble late today morning. Soldiers and police helped rescuers evacuate residents living along the mountain's fertile slopes, said Jimmy Eman, the acting mayor in the nearby town of Tomohon, adding that the only victim so far was the 56-year-old woman who died of a heart attack.
More than 6,000 people were crammed into schools, churches and other temporary shelters and authorities said 27,000 others living within 2 miles (3.5 kilometres) of the crater also would be moved.
"This is the largest eruption I've ever experienced," said Nelson Uada, who was among those evacuated overnight. "It was very scary. Glowing lava flowed like flames in the darkness and it sounded like we were in a war."
Flights to the nearest international airport in Manado, the provincial capital, were not disrupted, said Lucky Podaag, an airport spokesman.