Witnesses said a thick rain of volcanic ash from Mt Semeru was blotting out the Sun in two local districts.
No casualties have yet been reported and evacuations are under way, officials said.
Meanwhile a monitoring body issued a warning to airlines of an ash cloud rising up to 15,000m (50,000 ft).
The eruption took place at about 14:30 local time (07:30 GMT). Local authorities have set up a restricted zone of 5km (3 miles) from the crater after the eruption.
Thoriqul Haq, a local official, told Reuters that a road and bridge from the area to the nearby city of Malang had been severed.
“This has been a very pressing, rapid condition since it erupted,” he said.
The Volcanic Ash Advisory Center (VAAC) in Darwin, Australia said the ash appeared to have detached from the summit and was drifting south-west over the Indian Ocean.
The VAAC provides advice to the aviation industry about the location and movement of potentially hazardous volcanic ash.
Campbell Biggs, a meteorologist at the VAAC, told the BBC that the 15,000m plume was higher than the cruising altitude for most aircraft and would cause most flights in the vicinity to divert their flight paths to avoid it.
Ash that solidifies on cooler parts of plane engines can disrupt airflow, which can lead to engines stalling or failing completely.
It also affects visibility for the pilots and can affect air quality in the cabin – making oxygen masks a necessity.
The ash cloud should slowly dissipate, he said.
It is among Indonesia’s nearly 130 active volcanoes.
Indonesia is on the Pacific “Ring of Fire”, where continental plates meet causing frequent volcanic and seismic activity.