US president Barack Obama has been knocked off his feet by a pint-sized Superman.

Obama, whose eight year term will end with the swearing in of a new president in January, welcomed trick or treaters to the White House for the last time this week.

Among those who lined up to collect treats from Obama and his wife, First Lady Michelle Obama, were kids dressed as the presidential plane, Air Force One. talkshow host Jimmy Fallon, a Lame Duck and a pair of little President Obamas.

‚ÄčA small child dressed as Prince prompted Obama to burst into his own rendition of the late singer's hit Purple Rain, a video posted on YouTube by The White House showed.

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But it was a little boy dressed as Superman who captured Obama's heart, and knocked him off his feet.

Taking a high five from the tiny superhero, Obama purposefully fell backwards, telling the boy: "You are so strong. You knocked me over."

The boy, for his part, was also impressed.

"That's POTUS," he said in awe, using the acronym for the president.