Indonesian authorities raised the warning on Semeru volcano to the highest level on Sunday, Kompas TV reported, after an eruption spewed a column of ash high into the air.
The evacuation of people living near the volcano in East Java province had begun, a top regional administrator, Thoriqul Haq, told the broadcaster.
The plume from the volcano reached a height of 50,000 feet (15 km), said Japan's Meteorology Agency, which was monitoring for the possibility of a tsunami there.
Indonesia's Center for Volcanology and Geological Hazard Mitigation, PVMBG, raised the level of volcanic activity to IV from III, Kompas TV reported. The centre could not immediately be reached for comment.
The eruption on the eastern part of Java island, some 640 km (400 miles) east of the capital Jakarta, follows a series of earthquakes on the west of the island, including one last month that killed more than 300 people.
"Most of the road accesses have been closed since this morning. Now its raining volcanic ash and it has covered the view of the mountain," Bayu Deny Alfianto, a local volunteer told Reuters by phone.
Small eruptions were continuing and it was raining in the area, he said.
With the higher alert level, authorities warned residents not to conduct any activities within 8 km (5 miles) of Semeru's eruption centre, Kompas TV said.
The volcano began erupting at 2:46 a.m. (1946 GMT on Saturday), Indonesia's disaster mitigation agency, BNPB, said in a statement. Videos posted on social media showed grey ash clouds in nearby areas.
With 142 volcanoes, Indonesia has the largest population globally living in close range to a volcano, including 8.6 million within 10 km (6 miles).
The deadly late-November quake that hit West Java's Cianjur was a shallow temblor of 5.6 magnitude. A much deeper quake on Saturday in Gurat of 6.1 magnitude sent people running from buildings but did not cause major damage.