The heightened rumbling of Mount Talang coincided with a series of moderate earthquakes on Sumatra, which is still recovering from a massive 26 December quake and tsunami that killed nearly 130,000 people in Aceh province to the north.
"The status of Mount Talang is now at top alert," Surono, a volcanologist from the Directorate of Volcanology and Geophysics in the Java city of Bandung, said on Wednesday.
Local officials said 26,000 people had been evacuated from the slopes and areas around the volcano in West Sumatra province, adding the number was likely to rise.
Witnesses saw sparks of fire coming out of Mount Talang early on Wednesday morning.
The volcano is near the city of Padang, 938km northwest of Jakarta.
End of world
"It was like the end of the world," said Syafrudin, 65, a farmer, after fleeing from the mountainside and speaking outside a makeshift tent near the village of Tanjung Aua.
"The plates' movements release energy underground and the energy could add to the activity of many volcanoes on Sumatra"
Directorate of Volcanology and Geophysics volcanologist Surono
"I first heard the rumble and then the ground started shaking ... then there was smoke and sparks. We all ran in fear."
Volcanologists said they could not confirm whether lava had spewed out. Smoke had billowed up to 1km above the volcano's crater, and ash had travelled up to 12km away, Surono said.
"The activity has definitely stepped up and we have alerted airport officials to watch out for smoke," he added.
Scientists have warned of increased seismic activity in Indonesia as the plates that make up the earth's crust adjust after the magnitude-9 earthquake in December that triggered massive tsunami waves across the Indian Ocean.
"The plates' movements release energy underground and the energy could add to the activity of many volcanoes on Sumatra but it doesn't mean this will trigger an eruption," said Surono.
"Coincidentally, the pressure inside Mount Talang had been high and the quake activity has increased that energy," he added.
President Susilo Bambang Yudhoyono, who has urged Indonesians not to see the spate of quakes and the December tsunami as a sign of more natural calamities to come, will visit the area later on Wednesday, officials in Jakarta said.
Elvi Sahlan, deputy mayor of the town of Solok near the volcano, said many residents were frightened.
"Up till 2 o'clock this morning we have evacuated around 26,000 people from the surrounding areas and the number is likely to increase because there are many others who have not been evacuated," Sahlan said.
Indonesia has more than 100
Many residents spent the night in makeshift tents in open fields, while others took refuge at government buildings and sports stadiums in nearby towns.
"The whole family is stressed. It's better to be here now than to live in fear," said Yusmalidar, 40, who had just been evacuated in a government truck with his wife and three children.
One local official said there was a shortage of tents and water among residents.
The mountain first began rumbling on Tuesday.
Indonesia, and especially Sumatra, has been hit by daily aftershocks since the massive earthquake on 26 December.
A quake off Sumatra on 28 March killed at least 676 people, many on Nias island off Sumatra's west coast.
Indonesia is a vast archipelago of about 17,000 islands that is along the geologically active Pacific Ring of Fire and has more than 100 active volcanoes.