Mount Merapi, which looms on the horizon north of the major city of Yogyakarta, is one of Indonesia's most active volcanoes and lies in one of the world's most densely populated areas.
Some non-governmental relief agencies were poised to offer immediate help. One of them was World Vision Indonesia, a Christian relief and development agency.
"Right now, our biggest concern is the children," said Fadli Usman, World Vision Indonesia's rapid assessment team leader. "Children are always forgotten in the early moments of a disaster like this. My first task will be to assess the needs of the children and their families in the evacuation centers so our team can begin to help them."
Images on Indonesian TV showed the volcano lighting the sky with a fiery glow and sending thick clouds of gray smoke high into the air. The volcano has a summit elevation of nearly 10,000 feet (3,000 meters).