HONG KONG—The government’s first use of emergency powers in half a century failed to stop violence and vandalism flaring across the city, leaving officials with the choice of introducing more extreme measures to win back the streets or risking intervention from Beijing.

For the first time during the four months of unrest, uniformed soldiers from the Hong Kong garrison of the People’s Liberation Army raised a yellow warning flag at nearby protesters, saying: “You are in breach of the law. You may be prosecuted.”

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