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Two people died after their houses collapsed under the weight of ash, officials said. Some towns were said to be covered by 4cm (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 5cm on the runway and tarmac."
Residents wore masks to protect themselves from the dust and ash in the air
Around 200,000 people were ordered to evacuated their homes
This man became covered with ash as he travelled on the roads of Yogyakarta
Officials raised an alert on Thursday around an hour before the
volcano erupted, and urged around people living in 36 villages in a
10km (6 mile) radius around the volcano to evacuate.
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.
Covered in grey The volcano spewed ash and gravel that flew as far as 200km (124 miles) away, National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho told the BBC.
The nearest town from the volcano, Kediri, looks completely grey and white, the BBC's Alice Budisatrijo in Jakarta reports.
The thick dust makes it dangerous for pedestrians and vehicles to be on the road, and authorities say residents are volunteering to sweep the streets, our correspondent adds.
Kediri, 42km away from the volcano, looks completely grey and white
The 1,731-m (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.
The volcano last erupted in 1990, killing dozens of people. A powerful eruption in 1919 killed around 5,000 people.
Indonesia lies across a series of geological fault-lines and is prone to frequent earthquakes and volcanic eruptions.
There are around 130 active volcanoes in the country.
Earlier this month, Mount Sinabung on the island of Sumatra erupted, killing at least 14 people.
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Two people have been killed after their home collapsed from the weight of volcanic ash as Alice Budisatrijo reports.
Thousands of people are evacuating their homes in Indonesia after a volcano erupted in east Java.
Mount Kelud spewed ash and debris over a large area, including the city of Surabaya, about 130km (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 4cm (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 5cm on the runway and tarmac."
Residents wore masks to protect themselves from the dust and ash in the air
Around 200,000 people were ordered to evacuated their homes
This man became covered with ash as he travelled on the roads of Yogyakarta
Continue reading the main story
Recent Indonesian eruptions
- Mt Rokatenda on Palue island erupted in August, killing six people.
- Mt Sinabung on Sumatra island erupted in August 2010 after being dormant for 400 years. It killed 16 people in a fresh eruption in February 2014.
- Mt Merapi on Java island killed 353 people in a series of eruptions in 2010.
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.
Covered in grey The volcano spewed ash and gravel that flew as far as 200km (124 miles) away, National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho told the BBC.
The nearest town from the volcano, Kediri, looks completely grey and white, the BBC's Alice Budisatrijo in Jakarta reports.
The thick dust makes it dangerous for pedestrians and vehicles to be on the road, and authorities say residents are volunteering to sweep the streets, our correspondent adds.
Kediri, 42km away from the volcano, looks completely grey and white
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.
The volcano last erupted in 1990, killing dozens of people. A powerful eruption in 1919 killed around 5,000 people.
Indonesia lies across a series of geological fault-lines and is prone to frequent earthquakes and volcanic eruptions.
There are around 130 active volcanoes in the country.
Earlier this month, Mount Sinabung on the island of Sumatra erupted, killing at least 14 people.
Are you in Java? How have you been affected by the eruption of the volcano? Send your stories to Haveyoursay@bbc.co.uk with the heading Java.
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