Five knee-high androids have crossed the starting line in the world's first marathon for two-legged robots.

The contest is being held in the Japanese city of Osaka.

The contestants are expected to take four days to complete the course, which involves 423 laps of an indoor track.

Operators are allowed to change the robots' batteries and motors but if the machines fall over they must get up by themselves.

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Organisers say they expect the race to be won not by the fastest robot but by the one that can withstand the most wear and tear.