People have been relocated to 11 shelters on the island, Indonesia's disaster mitigation agency said, while search and rescue efforts were continuing on the island, about 640km east of the capital Jakarta.
One of the worst recorded eruptions of Semeru — the highest mountain on Java — was last year, killing 50 people and damaging approximately 3,000 homes and more than 20 schools.
The volcano has erupted about 55 times since the early 19th century, and while most of the eruptions have caused evacuations 11 recorded eruptions have led to deaths.
“Most roads have been closed since this morning and now it is raining volcanic ash and it has covered the view of the mountain,” community volunteer Bayu Deny Alfianto told Reuters from near the volcano.
Indonesia's Centre for Volcanology and Geological Hazard Mitigation, the PVMBG, raised the level of volcanic activity to its highest level and warned residents not to approach to within 8km of Semeru's eruption centre.
Hot ash clouds have drifted nearly 20km from the centre of the eruption, it said.
PVMBG chief Hendra Gunawan said a bigger volume of magma could have built up compared with previous eruptions of the volcano, in 2021 and 2020, which could mean greater danger for a larger area.
“Semeru's hot clouds could reach further and at a distance where there are many residences,” he said.
In a video sent to Reuters by police in the area, villagers were seen moving away from the slopes of the volcano, some with belongings stacked on motorbikes. A damaged bridge was covered in volcanic ash.
There are 142 volcanoes in Indonesia which has the world's largest number of people living close to a volcano, with 8.6 million people within 10km of one.