Mount Kelud spewed ash and debris over a large area, including the city of Surabaya, about 80 miles away.
Two people died after their houses collapsed under the weight of ash, officials said. Some towns were said to be covered by 1.6 in of ash.
Three major airports in Surabaya, Solo and Yogyakarta were forced to shut down because of low visibility.
There are also fears that debris could damage aircraft engines.
Andi Wirson, general manager at the Yogyakarta airport, said: “The current conditions are that volcanic ash is now covering the runway, apron and tarmac. We have already measured the thickness of the volcanic ash, which is at 5 cm on the runway and tarmac.”
Indonesia’s disaster agency on Friday said that more than 100,000 people had evacuated to temporary shelters.
Many in Indonesia are quite used to volcanic eruptions, and are reluctant to leave their homes and belongings, correspondents say.
Officials said two people died after their homes, which appeared to have weak structures, could not withstand the weight of gravel and ash, and caved in.
Some of the evacuees tried to visit their houses on Friday morning to gather clothing and valuables, but were forced to turn back by the continuous stream of volcanic ash and rocks from the volcano, AFP news agency reported.
The 5,600 ft Mount Kelud – which had been rumbling for several weeks – erupted on Thursday evening.
However, the head of Indonesia’s Volcano and Geology Agency, Hendrasto, who like many Indonesians goes by one name, said the eruption was gradually subsiding.
Experts say the volcano tends to quieten down after a large eruption, and more big eruptions are unlikely.
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