Mt Barujari in the Mt Rinjani National Park near Bali began spouting ash on Tuesday when over a thousand tourists were in attendance.
No casualties have been announced yet.
The eruption shot ash 6,500 feet into the air, interrupting flights at Lombok and Bali airports.
The national park is a popular destination for hikers and more than 400 tourists – foreign and local – had registered to hike the volcano.
The National Disaster Mitigation Agency said that a few hundred tourists are still missing who are possibly documenting this beautifully dangerous natural phenomenon.
Indonesia is home to about 130 active volcanoes and sits on the edge of the Pacific Ring of Fire.