HONGKONG takes on Vancouver today in the International Challenge to see who has the fitter and more active population. The aim is to encourage as many Hongkong people as possible to take part in 15 minutes of physical activity of their choice between midnight last night and 9 o'clock this evening. The Urban and Recreational Councils, which are arranging the event, have organised more than 470 free activities for the public to participate in. In addition, indoor games halls and sports grounds will be available for those who need specific facilities such as tennis and squash courts, or swimming pools. But not everyone who wants to take part has to use these facilities. The lone jogger, the housewife using keep-fit equipment at home, and the office worker exercising limbs rather than the brain will all be counted, as long as they have registered to take part in the Challenge Day. The competition works on an ''honour'' system, with no one to check whether you have carried out the exercises. The city which is able to encourage the biggest percentage of its residents to undertake 15 minutes exercise wins. Vancouver will have the edge on Hongkong. Hewlett-Packard will process Hongkong's results and, due to the time difference, Vancouver will have a rough idea of the number it has to beat by 6 am. This will give it time to stir its population into action. The International Challenge Day is being held under the auspices of the Trim and Fitness International Sport for all Association (TAFISA), which holds regular conferences and seminars. Previously, there have been competitions between individual cities - Dortmund beat Ballarat last year - but this is the first occasion that the event has been held worldwide. Thirty-four cities in 20 countries are taking part this year. Cities in Israel, Iceland, Romania and England will be challenging cities in Portugal, Canada, Brazil and Wales. Carrying the flag for Asia will be: Seoul and Pusan, South Korea; Phuket, Thailand; Ballarat and Melbourne, Australia; and Hongkong. Hongkong was drawn against Vancouver in January this year. The event was formalised in February when the mayor of Vancouver, Mr Gordon Campbell, flew to Hongkong to sign an agreement with the chairmen of the Urban and Regional Councils. ''Although Canada has much experience in organising Challenge Day programmes, Hongkong will do its best to beat Vancouver,'' said Mr Cheung Yan-lung, chairman of the Regional Council, at the signing ceremony. Despite being a friendly event, there will be more than a touch of rivalry, given the strong links between Hongkong and Vancouver. The city flag of the winner will be flown in the City Hall of its opponent for 14 days after the event. To encourage people in Hongkong to participate, those who register qualify for a lucky draw, which will take place on Friday. Prizes include an eight-day package tour to South Africa, two return tickets to Vancouver, bank savings accounts, a refrigerator, a washing machine, a driving instruction course, sewing machines and dinners for two at some of Hongkong's leading hotels. The Challenge Day will be a fun day for Hongkong, but there is also a serious motive. As well as promoting the many sporting facilities of the Urban and Regional Councils, the main message is that sparing 15 minutes a day to get fitter is easy. ''No matter where you are, it is always possible to set aside 15 minutes each day,'' said Ms Lorraine Mao, chief recreation and sports officer of the Urban Services Department. ''Many people walk or go jogging and, even in the office, there are simple exercises that people can do. ''We want to spread the word to Hongkong citizens that it is easy to start with 15 minutes, and, then, as people get fitter, they can take up hobbies such as tennis and swimming.'' The real message behind this exciting event is that health is the most valuable asset a person has, and that keeping fit is an important asset of a healthy lifestyle. The organisers hope the event's message will not be forgotten tomorrow, and that people who take part will be more aware of the importance of healthy pursuits. ''Both Mr Cheung and I hope that members of the public will try their best in this health-orientated activity so that we will win the competition and bring credit to Hongkong,'' said Dr Ronald Leung Ding-bong, chairman of the Urban Council. Even if Hongkong does not win, the real winner will be a healthy population.