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.
Reuters Television footage showed vast black clouds billowing into the sky, against a background of trees and farmhouses.
A Reuters TV producer said it began raining at around 4 p.m. and that clouds obscuring the 2,460-metre volcano's peak made it difficult to tell if ash was still erupting in the afternoon.
The air is very foggy and smoky, he said. There is no one around, only police and officials.
About 21,000 people had been evacuated. Displaced residents, including children wearing masks, milled about in a makeshift reception centre 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 ORDERS MEASURES
President Susilo Bambang Yudhoyono instructed the disaster mitigation agency to help set up emergency tents, kitchens and toilets, said presidential spokesman Julian Pasha.
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.
Surono, head of Indonesia's volcanologist centre, 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, 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 radius of the volcano, volcanologist 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 neighbouring countries.
Here, it is three millimetres thick on the leaves of plants, he said.
It has the potential to damage people's respiratory tracts, but in my lifetime of studying volcanoes I have never heard of anyone dying from inhaling volcanic ash.
Andang Santoso, a spokesman for the government air agency PT Angkasa Pura II, said major flight itineraries were unaffected.
However, there have been delays on the route between Medan and Sibolga, a town south of the volcano, he told Reuters in a telephone text message.
(Reporting by Sunanda Creagh and Olivia Rondonuwu; Editing by Sara Webb and Ron Popeski)