The alert was being maintained at the second-highest level and authorities told people to stay out of the existing 5-kilometer danger zone around the crater as the local administrations in Central Java and Yogyakarta provinces closely monitor the situation.
The 2,968-meter (9,737-foot) volcano is on the densely populated island of Java and near the ancient city of Yogyakarta. It is the most active of dozens of Indonesian volcanoes and has repeatedly erupted with lava and gas clouds recently.
Merapi's last major eruption in 2010 killed 347 people.
Indonesia, an archipelago of 270 million people, is prone to earthquakes and volcanic activity because it sits along the Pacific “Ring of Fire,” a horseshoe-shaped series of seismic fault lines around the ocean.