Bangladesh has ordered an emergency deployment of 100 mobile toilets in its capital to head off a worrying rise in public defecation, Dhaka's mayor said Tuesday.

Sadeque Hossain Khoka told AFP the sprawling metropolis, which has an official population of 12 million, has only 48 public toilets - one for every quarter of a million residents.

This cannot meet demand, he said, leading to a pile up of the kind of stools it would be inadvisable to sit on in parks and by roadsides.

"We have launched 100 mobile toilets, which will be carried around manually on tricycle vans. They will be strategically placed so that people don't have to use road corners to answer the call of nature," he said.

Unofficially, Dhaka's population tops 20 million, when vast slums on the city's outskirts are included.

The tin-sided mobile toilets are plastered with colourful advertisements including quotes from a famous Bangla poem which tells people: "Let's do good work, no matter where you were born."

They also carry posters urging people not to treat streets and open spaces as public toilets.

The mobile toilets will charge five taka (3.5 cents) for people to defecate and two taka to urinate, and are now available for 12 hours a day - between 8am and 8pm.

Khoka said the authorities would introduce more mobile toilets if the venture was a success.

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