Other videos shared by emergency officials showed residents running as towering plumes of smoke and ash blanketed nearby villages in East Java province.
The deputy chief of Lumajang district, Indah Masdar, told a news conference at least one person had died and 41 people had suffered burn injuries.
Authorities were setting up evacuation tents, the country's disaster mitigation agency (BNPB) said.
Semeru, the highest on Java island, is among Indonesia's nearly 130 active volcanoes. It erupted in January, causing no casualties.