The formal threat level has been raised from a three to a four, which is the highest.
This means that inhabited areas are in the risk zone and that the volcano's activity has been stepped up, a spokesperson for Indonesia's responsible authority explains to the television channel Kompas.
The eruption was caused by monsoon rains that fell on the volcano and caused a lava crust on the summit - 3,676 meters above sea level - to erode, according to the authority.
Columns of ash have shot up over a kilometer in height.
Buildings in several surrounding villages have been covered in a light layer of ash, and lava flows down the mountain in torrents.
People are advised to stay at least half a mile away from the volcano's crater.
A few hundred people have been temporarily evacuated.
There are no reports of personal injuries.
Semeru's last major eruption occurred almost exactly one year ago.
Then 51 people died in villages that were buried in mud masses as a result of landslides.
Semeru is located on the eastern part of Indonesia's densely populated main island of Java.
The western part has been hit by earthquakes in recent weeks.
In Japan, there are warnings that high waves may hit islands in southern Okinawa as a result of the volcanic eruption.