There were no immediate reports of casualties, the country's disaster mitigation agency said.
Images broadcast on local outlet Kompas TV showed ash-covered houses and roads at a village near the volcano, located on Java Island, near Indonesia's cultural capital Yogyakarta.
The Merapi Volcano Observatory estimated the ash cloud reached 9,600 feet (3,000 metres) above the summit.
Authorities established a restricted zone of seven kilometres from the crater after the eruption, which was recorded at 12:12 pm (0512 GMT).
"To anticipate potential danger from Mount Merapi eruption, the public is advised to stop any activities in the potential danger area," agency spokesperson Abdul Muhari said in a statement on Saturday.
Nearby residents should also expect "disruptions" from ash and be aware of potential dangers from volcanic mudflow, particularly if it rains near the volcano, Muhari said.
At least eight villages near the volcano have been affected by volcanic ash, an officer at one of Merapi's observation posts said in a statement.
The volcano's last major eruption in 2010 killed more than 300 people and forced the evacuation of some 280,000 residents.
It was Merapi's most powerful eruption since 1930, when about 1,300 people were killed. An eruption in 1994 left about 60 people dead.
The volcano's alert status has remained at the second-highest level since 2020 after showing renewed activity.
Indonesia, which has nearly 130 active volcanoes, sits on the Pacific "Ring of Fire", where the meeting of continental plates causes high volcanic and seismic activity.
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