Residents in three districts in a 6-kilometer radius danger zone of the volcanic Mount Sinabung have been evacuated to Kabanjahe and Brastagi prior to the eruption in trucks and ambulances.
Many had left their homes even before they were evacuated.
They have been placed in eight shelters, including the regency administration building and other public places.
Evacuees are from 18 affected villages.
The number of people living in temporary shelters had since swelled by 3,000 to 21,000.
Sixteen shelters have been set up to accommodate people who evacuated their villagers as ash and stones fell around the area early on Sunday.
“We were shocked because the local administration had previously informed us that the volcano was stable. We panicked and fled our homes,” Sukanalu subdistrict head Paten Sitepu said.
Lulled into a sense of calm by news of the stable condition, some residents who had fled their houses two days ago when the volcano first started spewing thick smoke decided to return home only to leave their houses again a few hours later when the volcano erupted.
The Sinabung in Karo district in N Sumatra. Photo: AFP (above) and The Jakarta Post (map)
Evacuees saw the volcano was spewing hot lava from its crater and send smoke and dust 1,500 meters into the air.
The area was blanketed with thick black smoke, small stones and there's a strong smell of sulphur.
Authorities had anticipated them to have respiratory problems from inhaling dust.
“I have a constant headache and stomachache,” Maranata, an internally displaced person, told Metro TV.
Local media reported that a man had died after experiencing breathing difficulty while fleeing from his village, but Karo district official Andes Mbaga told AFP, N. Surbakti of Cinta Rakyat subdistrict reportedly had died from heart failure.
'He had a history of heart problems. He was having breathing problems which could have been worsened by the ash and dust floating around,' he said.
Reuters reported another death from a heart attack.
As of Sunday afternoon, 50 tons of rice and 12,000 face masks have been supplied to temporary shelters.
After no volcanic activity for more than four centuries, Mount Sinabung in Karo regency, North Sumatra, erupted early Sunday morning, 2 minutes past midnight with three explosions followed by a lava flow down the slope of the volcano, which is 2,460 meters above sea level.
Sinabung had shown 'some volcanic activity' since Friday. The ash has spread to a distance of 30 kilometres from the volcano prior to the eruption. Many of the villagers evacuated were farmers and they said the ash had settled on their vegetable farms.
Volcano spewed a vast cloud of smoke and ash high into the air on Monday, disrupting flights and sending thousands more people into temporary shelters, officials said.
This was the first eruption at Mt. Sinabung since 1600.
And it erupted again for the second day after springing to life for the first time in four centuries on Sunday.
'It erupted again at 6.30am (2330 GMT, 7.30am Singapore time) and lasted about 15 minutes. The smoke and ash reached at least 2,000 metres,' government volcanologist Agus Budianto said.
The eruption was bigger than on Sunday when Sinabung rumbled into action for the first time since 1600, adding its name to the list of 69 active volcanoes in the massive South-east Asian archipelago.
“For almost 410 years the volcano showed no dangerous volcanic activity, which was why it was categorized as a dormant or type B volcano,” Kaban told The Jakarta Post.
The Volcanology and Geological Disaster Mitigation Center (PVMBG) announced that the eruption set a historical precedent.
“This is the first time in our history that a type B volcano suddenly turned into a dangerous one, or a type A volcano,” PVMBG head Surono said after meeting with provincial officials Sunday.
The center classifies type A volcanoes as having erupted at least once since 1600, type B as never having erupted since 1600, but showing signs of volcanic activity, and type C as never having erupted in recorded history.
Surono said the PVMBG was studying why Sinabung erupted.
“We never monitored the development of Mt. Sinabung because it was considered extinct,” Surono said.
He added he would increase the monitoring activities on the volcano.
AFP reported Indonesia sits on the Pacific 'Ring of Fire', where the meeting of continental plates causes high volcanic and seismic activity.
It has more active volcanoes than any other country.
The eruption also led to the delay of several flights at Medan’s Polonia airport as airlines were warned to avoid remote Mount Sinabung in northern Sumatra.
Transport ministry spokesman Bambang Ervan said. 'It may affect flight traffic to and from the province. It all depends on the direction of the wind,' he told AFP.
Several domestic flights had to be cancelled on Sunday due to the smoke.