COVID-19　threatens　the　world.　Social　distancing　is　a　significant　factor　in　determining　the　spread　of　this　disease,　and　social　distancing　is　strongly　affected　by　the　local　travel　behaviour　of　people　in　large　cities.　In　this　study,　we　analysed　the　changes　in　the　local　travel　behaviour　of　various　population　groups　in　Hong　Kong,　between　1　January　and　31　March　2020,　by　using　second-by-second　smartcard　data　obtained　from　the　Mass　Transit　Railway　Corporation　(MTRC)　system.　Due　to　the　pandemic,　local　travel　volume　decreased　by　43%,　49%　and　59%　during　weekdays,　Saturdays　and　Sundays,　respectively.　The　local　travel　volumes　of　adults,　children,　students　and　senior　citizens　decreased　by　42%,　86%,　73%　and　48%,　respectively.　The　local　travel　behaviour　changes　for　adults　and　seniors　between　non-pandemic　and　pandemic　times　were　greater　than　those　between　weekdays　and　weekends.　The　opposite　was　true　for　children　and　students.　During　the　pandemic,　the　daily　commute　flow　decreased　by　42%.　Local　trips　to　shopping　areas,　amusement　areas　and　borders　decreased　by　42%,　81%　and　99%,　respectively.　The　effective　reproduction　number　(Rt)　of　COVID-19　had　the　strongest　association　with　daily　population　use　of　the　MTR　7?8　days　earlier.