By Alastair Wanklyn
The tsunami struck in the evening local time, shortly after a volcano erupted offshore. The activity on Krakatoa island either pushed up the sea bed or triggered underwater landslides, sending a wall of water towards the coast.
The wave struck seafront towns and villages on the islands of Java and Sumatra. One survivor said he clung to a fence as the water surged past. He feared he would be swept out to sea when the tsunami receded.
A spokesman for Indonesia's disasters agency was quoted saying Indonesia lacks a warning system for volcanic eruptions. There was no alarm, he said, because there was no earthquake. Tsunami sensors are located where tectonic plates meet.
Indonesia regularly suffers natural disasters. In September an earthquake struck the island of Sulawesi, killing more than 2,000 people.
And Krakatoa, the volcano that erupted on Saturday, killed tens of thousands of people when it first erupted, in the 1880s.
International aid agencies said they are sending food and shelter materials.
But for residents the threat is not over yet. Indonesia's meteorological agency said the volcano on Krakatoa is still erupting, and further tsunamis are possible.