On the day of the National Day parade, the whole capital poured into the streets.
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From Su Shi's poem 《八月十七复登望海楼》: 赖有明朝看潮在,万人空巷斗新妆 - describing Hangzhou residents pouring out to watch the tidal bore. 巷 ('alley') empties because everyone has rushed out to see something.
mistakes
Common misunderstanding: many readers picture 'empty streets' (nobody there). Correct image is the opposite - the ALLEYS are empty because everyone left them to crowd somewhere else. Always describes a hugely popular event, never a ghost town.