Mount Sinabung, in the north of the island of Sumatra, began erupting around midnight after rumbling for several days, prompting some villagers to panic before the mass evacuation got under way.
Mount Sinabung last erupted in 1600, so observers don't know the volcano's eruption pattern and are monitoring it closely for more activity.
Evacuations on the volcano's slopes started Friday at the first signs of activity. Up to 10,000 people who fled are staying in government buildings, houses of worship and other evacuation centers in two nearby towns.
The government has distributed 7,000 masks to refugees and set up public kitchens so people can cook food, said Priyadi Kardono, spokesman for the National Disaster Management Agency.
Indonesia is on the so-called Pacific Rim of Fire, an arc of volcanoes and geological fault lines triggering frequent earthquakes around the Pacific Basin. The eruption triggered the highest red volcano alert.
Two people died, one from breathing problems and the other from a heart attack, and two suffered injuries in road accidents as trucks, ambulances and buses were mobilized in the rescue operation.
Authorities took at least 12,000 people from high risk areas on the slopes of the 2,460-meter volcano to temporary shelters. Local TV showed showed women and children wearing face masks in cramped tents.
The area around the volcano is largely agricultural.
"Since this is the first eruption we've had in Sinabung, we're anticipating residents will remain at the shelters for at least a week while waiting for further status alert," said Priyadi Kardono.
The eruption has not damaged roads or bridges. The nearest big city is Medan where there were no disruptions to any of the air flights.
Second EruptionThe volcano, Mount Sinabung, erupted again on Monday, pitching ash two km (1.5 miles) into the air and sending nearby residents scurrying from their homes.
Villages were emptying fast near Mount Sinabung on the north of Sumatra island, leaving behind only officials from the bureau of meteorology and the police. Short-haul flights skirting the volcano were delayed.
About 21,000 people had been evacuated. Displaced residents, including children wearing masks, milled about in a makeshift reception center with a roof but no walls.
Satebi Ginting, a vegetable farmer who fled her village to shelter in the nearby town of Brastagi, said she did not know when she would return home.
"I am still too scared to go back," she said in a camp hosting around 400 people, where a band was playing traditional local tunes.
President Susilo Bambang Yudhoyono instructed the disaster mitigation agency to help set up emergency tents, kitchens and toilets, said presidential spokesman Julian Pasha.
Surono, head of Indonesia's vulcanology center, earlier said Monday's eruption was more powerful than the first a day earlier.
"Earlier today was another eruption at 6.30 a.m., sending out smoke as high as two km, more or less," he told Reuters.
A Reuters photographer said he saw plumes of smoke rising from the volcano after the second eruption. Inactive since 1600, it had been rumbling for several days.
"I saw some hot pieces of volcanic rock come out and burn trees in the area," he said. A smell of sulphur pervaded the air as residents moved out of their homes to temporary shelters.
Many residents fled to Medan, 50 km (30 miles), Indonesia's third-largest city, northeast of the volcano. Officials said much of the movement was unnecessary.
"People have been evacuated from areas within a six km (four-mile) radius of the volcano," vulcanologist Surono said. "Beyond six km it is safe, but there has still been a lot of panic among people here who don't understand that."
He said it was impossible to know when the eruptions would stop, but it was unlikely volcanic dust would drift to neighboring countries. Air flights continue to be unaffected.
Reuters, "Thousands flee as long-sleepy Sumatra volcano erupts", accessed August 30, 2010
Reuters,"Indonesian volcano erupts again, many evacuated", accessed August 30, 2010