Locals call it 'neiyi wai chuan' or wearing your underwear on the outside. Some say the city is their home and they can wear anything at home.

'It's very comfortable. I wear pyjamas when I go outside to places within walking distance,' said clerk Xu Jun, wearing a blue and white striped ensemble as he shopped at a local market.

To officials, it's an affront to decency and a potential pitfall in Shanghai's bid to host the World Exposition in 2010.

'This is coarse and uncivilised,' the conservative Wen Hui Bao newspaper said in a front-page editorial. 'The bearing of these people has spoiled the view of an international city.'

The popular Xinmin Evening News sounded a hopeful note, saying more people were starting to wear appropriate clothing outside.

First-time visitors to the city may wonder if there has been a mass break-out from a mental health institution.

No, it's only residents asserting the right to parade outside dressed in outrageous patterns. Officials have never conducted a census of what newspapers call the pyjama clan, but sightings of its members are common. Editorials lament the practice is preventing Shanghai from international city status.

The origins of the latest campaign have their roots in competition with capital Beijing, which has the 2008 Olympics. Shanghai is vying for the 2010 Expo.

Beijing has a campaign to get the best of the capital's manhood to wear T-shirts in the summer heat and cover up their bellies. Not to be outdone, Shanghai has the pyjama police.

However, officials may be fighting a losing battle in Shanghai. As the city moves to upgrade dilapidated housing, many old homes lack air-conditioning, making pyjamas a cool alternative.

During the 1966-1976 Cultural Revolution - when everyone essentially wore the same thing - Shanghai women found ways to stand out from the masses with more flattering cuts to their Mao suits.

Shopkeepers say some women now buy nightgowns specifically with the idea of wearing them outside the house.

The Wen Hui Bao says, with some regret, that it is impossible to ban people from hitting the pavement in pyjamas.

Here are a few suggestions for how Shanghai might use persuasion and propaganda to make the latest campaign as effective as the movement to eradicate the 'four pests' in the 1950s, when the country mobilised to kill flies, mosquitoes, rats and sparrows.

The city could declare certain areas 'no nightwear' zones, similar to moves to ban motorcycles from certain streets under new rules announced two weeks ago. 'No pyjamas' could become the eighth deadly sin, joining the city's list of seven points of objectionable behaviour. The list includes swearing, spitting, jaywalking, littering, damaging public property, destroying the environment and smoking in public.

And if Shanghai is serious about ensuring its place as an international city, there are other goals that are certainly as worthy as keeping naughty nighties off the streets.

Shanghai has announced plans to become an international financial centre in 10 to 20 years. While the government gave no hint of what this means, bankers said free movement of capital and equal treatment for foreign and local financial institutions were the keys to becoming a global financial hub.

Shanghai might try to give a bit more space to its artists to ensure they don't flee to Beijing, which is still seen as the nation's centre of cultural creativity.

And a good brass section for its orchestra would also help Shanghai's emergence as an international city.

Three years ago, Shanghai's No 2 orchestra received funds to hire foreign musicians to improve its sound, after a government official complained about a local trumpet player botching a solo in Swan Lake.

The Shanghai Broadcasting Symphony Orchestra recently sacked three foreign musicians after a contract dispute while five others pointedly declined to extend their terms. The embarrassing incident sounded a sour note in Shanghai's efforts to raise its cultural status.

Perhaps Shanghai could turn the pyjama phenomenon inside out and embrace the quirky custom.

It could turn the pyjama game into a tourist attraction, a symbol of the city's unique character. Millions of tourists have visited another local eyesore - the futuristic Oriental Pearl Television tower - now a beloved if slightly bizarre emblem of Shanghai.

With the razing of so many old buildings across the city in the rush to modernise, Shanghai needs to preserve some of its heritage.