A toddler has delighted Barack Obama by dressing as a 'Mini Pope' complete with his own popemobile at a White House Halloween party, causing the US president to immediately award the boy top prize for best costume.

Mr Obama and the First Lady were handing out lollies on the South Lawn of the White House on Friday when the pint-sized pontiff rocked up, pushed in a replica popemobile stroller.

"Top prize!" the president said while laughing, using his discretion as the world's most powerful man to dish out the gong.

The White House South Lawn was transformed into an "enchanted forest" for the annual event, with hundreds of schoolchildren turning up for trick-or-treats from Obama, who notably did not dress up.