An AFP correspondent at the scene said some locals reported hearing an explosion before panic broke out, but the explosion could not immediately be confirmed with scientists.
One resident, Yono, was evacuating on his motorcycle with his wife and son, carrying a big bundle.
"We panicked, Kelut has erupted," he said as he rushed downhill.
Tens of thousands of people have been evacuated from the fertile slopes of the historically deadly volcano, but an unknown number have refused to budge, evading patrolling police and soldiers.
Mount Kelut has been on high alert for more than two weeks but activity escalated dramatically on Friday, triggering fresh rounds of evacuations carried out by troops and local officials using hundreds of trucks.
About 130,000 people live within a 10km danger zone around Mount Kelut, according to the health ministry.
Since record-keeping began, Mount Kelut's eruptions have claimed more than 15,000 lives, including an estimated 10,000 in a catastrophic 1586 eruption. A 1919 eruption spewed heat clouds that killed 5,160 people.