Indonesia's most volatile volcano started erupting Tuesday, after scientists warned that pressure building beneath its dome could trigger the most powerful eruption in years.
Up to 20 people were injured by hot ash spewed from Mount Merapi, said an AP reporter who witnessed them being taken away for treatment.
Subandriyo, chief vulcanologist in the area, said the eruption started just before dusk Tuesday. The volcano had rumbled and groaned for hours.
"There was a thunderous rumble that went on for ages, maybe 15 minutes," said Sukamto, a farmer who by nighfall had yet to abandon his home on the mountain's fertile slopes. "Then huge plumes of hot ash started shooting up into the air."
Scientists have warned that pressure building beneath Merapi's lava dome could trigger one of the most powerful blasts in years.
"The energy is building up. ... We hope it will release slowly," government volcanologist Surono told reporters. "Otherwise we're looking at a potentially huge eruption, bigger than anything we've seen in years."
The alert level for the 9,737-foot (2,968-metre) mountain has been raised to its highest level.
Some 11,400 villagers on the mountain were urged to evacuate. But most who fled were the elderly and children, while adults stayed to tend to homes and farms on the mountain's fertile slopes.
In 2006, an avalanche of blistering gases and rock fragments raced down the volcano and killed two people. A similar eruption in 1994 killed 60 people, and 1,300 people died in a 1930 blast.
Indonesian officials were also trying to assess the impact of a 7.7-magnitude earthquake late Monday that caused a tsunami off western Indonesia, leaving scores of villagers dead or missing.
Indonesia is a vast archipelago of 237 million people, and the volcano and earthquake epicenter are about 8,000 miles (13,000 kilometers) apart. The nation is prone to earthquakes and volcanic activity due to its location on the so-called Pacific Ring of Fire — a series of fault lines stretching from the Western Hemisphere through Japan and Southeast Asia.
There are more than 129 active volcanoes to watch in Indonesia, which is spread across 17,500 islands.