BEIJING—China broke a period of relative silence after a decisive loss by Beijing’s allies in Hong Kong’s local elections, blaming the result on foreign interference and offering few signs the landslide victory by pro-democracy candidates is prompting any change in its Hong Kong strategy.

China, speaking through state media, decried the election as a sabotage campaign orchestrated by antiestablishment forces, but it offered little evidence to back its claim. It hewed to earlier rhetoric that other countries, particularly...