The ceremony in Kathmandu, where the black-clad, sunglasses-wearing 35-year-old took center stage, included cheering crowds and praying Buddhist monks.
Shortly before that, Shah had commented on the election victory on social media: "The strength of unity is my national strength," he wrote in a post that received over 3 million views in a few hours.
The election was held after this autumn's deadly "Gen Z protests" against corruption and plans for a social media ban, which led to the fall of the previous government.
Shah, who had been Kathmandu's mayor, played a leading role in the protests.
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