Ashes mixed with sand rained over villages at a distance of up to 10 kilometers (six miles) from the belching crater near the cultural capital of Indonesia, Yogyakarta.
"There was a thunderous noise for at least five minutes and I could see the ash clouds of my house," Jarmaji, a resident of the Boyolali regency, told AFP.
The authorities did not raise the alert status of the volcano, but temporarily closed the international airport of the city of Solo, also known as Surakarta, about 40 kilometers away after the early morning eruption.
The Indonesian volcano agency warned residents to stay out of a prohibited area three kilometers around Mount Merapi, citing the possible danger of lava flow and pyroclastic flows, a mixture of rapid movement of hot gas and volcanic material.
The last major eruption of Mount Merapi in 2010 killed more than 300 people and forced the evacuation of some 280,000 residents.
That was Merapi's most powerful eruption since 1930, which killed some 1,300 people, while another explosion in 1994 claimed about 60 lives.
The Southeast Asian nation, an archipelago of more than 17,000 islands and islets, has about 130 active volcanoes.
It sits in the Pacific "Ring of Fire", a vast area of geological instability where the collision of tectonic plates causes frequent earthquakes and significant volcanic activity.