US President Barack Obama urged young people not to "pull back" from the world and to reject cynicism and xenophobia during a Q&A in London.
Obama's comments came after he faced criticism from leading Brexit campaigners, who branded him an irrelevant "lame duck", over a speech he made supporting the UK's membership of the EU.
Boris Johnson has said it is "perverse" and "hypocritical" for the US to urge Britain to stay in the EU when it would never agree to cede power to a similar bloc.
The London Mayor said: "It is very odd, it is perverse, it is hypocritical."
He said that president Barack Obama "has a perfect right" to speak out but suggested he may not understand the nature of the EU.
Mr Johnson also denied that he was looking isolated in his pro-Brexit stance.
He suggested the government was "ringing round every other friendly country" in order to drum up a public show of support for their membership.
"They are saying look we're in a bit of a spot, can you say something positive about our membership of the European Union?" he added.
Barack Obama has warned that Britain's influence on the global stage would be diminished if it votes to leave the EU.
The presidential hopeful says she agrees with Obama comments adding that she 'has always valued a strong UK in a strong EU'.
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