This heightened fears of an eruption just days after a devastating earthquake that struck nearby Yogyakarta.
Smoke was seen rising up to 900 metres into the air. The area remains on high alert for a possible eruption.
Merapi which means mountain of fire had calmed down for a few days, but seems to have been affected by Saturday's strong quake, which killed more than 5,400 people in central Java.
Photograph: Mike Clarke/AFP/Getty Images