Vulcanologists had warned for the previous two days that the mountain, with a history of violent explosions, was coming back to life.
Tourists and residents were being ordered to remain outside a 2-mile exclusion zone as a precautionary measure.
Lokon’s last eruption was in 1991, when a Swiss hiker was killed by the blast and thousands of villagers were forced from their homes.
Nearby Mount Soputan erupted in early July, spewing ash and smoke 16,000 feet into the atmosphere.
Photo: Indonesian Volcanology Survey