Scientists raised the alert at Mount Kelud to the highest level earlier this week, pointing to rising temperatures in the lake of its crater and deep underground tremors. Authorities ordered 116,000 people living along the fertile slopes to evacuate, but many have refused, saying they need to tend to their crops and animals.
"If we didn't force them - in this case with a showing of firearms - the villagers would not budge," said local police chief Col. Tjuk Basuki, adding that residents have been repeatedly warned about the dangers of the volcano. "We had no choice but to do this for their safety."
Mount Kelud, located on densely populated Java island, last erupted in 1990, killing dozens. In 1919, a powerful explosion, heard hundreds of miles away, destroyed dozens of villages and killed at least 5,160 people.
Volcanic activity appeared to stabilize Friday, with no spikes in temperature and a reduction in the number, but not intensity, of underground tremors, said a senior government vulcanologist. But he noted that a similar pattern emerged days before the 1990 eruption.
"This is exactly the situation we are most afraid of," said Surono, who goes by only one name. "Last time, the volcano stabilized and ... then suddenly erupted. I can't tell what is going on inside Kelud ... but anything could happen anytime now."
Thousands of people have left the mountain, many settled in temporary shelters along its base. Some held Islamic prayers beneath a tent Friday, where a preacher told them to remain calm and follow the advice of authorities when the eruption finally comes.
"We pray to Allah to protect us always," Abdul Zukri told them.
Among those who had refused to leave a 6 mile danger zone around the mountain were 80-year-old Kamisah and her handicapped son Yassir, who survived the last blast after shutting the doors and windows and hiding inside.
"Whatever happens, we put our lives in God's hands," she said Wednesday from their hillside farmhouse where the family collects hay to sell to local livestock farmers. "We were shaking with fear, but we were lucky."
The U.N. said in a statement Friday the World Health Organization had activated 100 medics, put 200 health facilities on alert and established 41 outreach health posts. Emergency health kits, masks and essential equipment also were distributed.
Indonesia, which has about 150 active volcanos spread across 17,500 islands, sits on the so-called Pacific "Ring of Fire" - a series of volcanos and fault lines stretching from the Western Hemisphere through Japan and Southeast Asia.
Mount Kelud is 385 miles east of the capital, Jakarta.
The 1,731-meter volcano normally erupts without warning signs, like smoke or ash. Its explosive activity typically starts with a steam blast - when surfacing magma meets ground water - producing hot mud flows and pyroclastic surges.
Scientists say the temperature of a crater-lake formed in the massive 1919 blast is rapidly heating up and hundreds of volcanic earthquakes deep inside the mountain - far more than preceded the eruption 17 years ago - have been recorded in recent days.
In 1919, the flows traveled 24 miles in less than an hour, devastating vast tracts of farmland. A drainage system, established by the Dutch in the 1920s, maintains low water levels in the crater lake aimed at reducing damage.