A man looks on as Mount Semeru releases volcanic materials during an eruption on Sunday, Dec. 4, 2022 in Lumajang, East java, Indonesia. AP
The eruption of the highest mountain on Indonesia's main island of Java, around 800 kilometres (500 miles) southeast of capital Jakarta, prompted authorities to raise the alert status to the highest level.
"Hot avalanches" caused by piles of lava at the tip of the 3,676-metre (12,000 feet) volcano slid down after the eruption, National Disaster Mitigation Agency (BNPB) spokesperson Abdul Muhari said in a statement.
The increased threat level "means the danger has threatened the people's settlement and the volcano's activity has escalated", Volcanology and Geological Disaster Mitigation Center (PVMBG) spokesperson Hendra Gunawan told broadcaster Kompas TV.
No casualties or injuries were reported immediately after the eruption but Gunawan warned nearby residents not to travel within eight kilometres (five miles) of the crater after the threat level was raised to four.
The BNPB said 1,979 people were taken to 11 shelters, with at least 6 villages affected by the eruption.
Images on local TV showed evacuees, mostly women and children, taking shelter in a school.
Videos shared with AFP by local rescue group Irannala Rescue showed a huge black cloud rising from the volcano's crater, engulfing the sky and blocking the sun in nearby villages.
The villages were being battered by monsoon rains by the afternoon and the rainfall was mixing with volcanic ash, according to Kompas.
Residents were also told to avoid a southeastern area 13 kilometres (8 miles) along a river in the direction where the ash was travelling.
"A lot of people have started to go down," Thoriqul Haq, the local administration chief for Lumajang, where the volcano is located, told Kompas TV.
Most residents in the two most-threatened villages have evacuated themselves, said Patria Dwi Hastiadi, spokesperson of the Lumajang Disaster Mitigation Agency.
Japan's weather agency had earlier warned that a tsunami was possible in the southern islands of Miyako and Yaeyama in Okinawa prefecture, Kyodo news agency reported, but the country's meteorological agency said no significant tidal changes were observed.
"It's getting dark"
The internet in the area around Semeru was cut and phone signals were patchy after the eruption, according to an AFP journalist.
The local rescue agency distributed free masks to the public because of the threat of polluted air to vulnerable residents.
As rescuer Gunawan, who has the same name as the PVMBG spokesperson, filmed clouds above him in East Java after the eruption, a video showed the sky quickly turning to black as a colossal plume of ash engulfed any light.
"I've promised you, we won't miss it again," he told residents in a local Javanese dialect, appearing to refer to last year's eruption before flicking a peace sign to the camera.
Another rescuer filmed an ominously dark scene outdoors at midday, which could have been midnight.
"It's getting dark, bro," he said to the camera, as a seismograph whistled in the background.
Mount Semeru last erupted exactly one year ago, killing at least 51 people and damaging more than 5,000 homes.
The disaster left entire streets filled with mud and ash that swallowed houses and vehicles, with nearly 10,000 people seeking refuge.
A bridge that connects two districts in the area that was being rebuilt after last year's eruption was heavily damaged again on Sunday, according to a PVMBG media officer.
Semeru's alert status had remained at its second-highest level since its previous major eruption in December 2020, which also forced thousands to flee and left villages covered.
Indonesia sits on the Pacific Ring of Fire, where the meeting of continental plates causes high volcanic and seismic activity.
The Southeast Asian archipelago nation has nearly 130 active volcanoes.
A volcano in the strait between Java and Sumatra islands erupted in late 2018, causing an underwater landslide and tsunami that killed more than 400 people.