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.
David Cameron and Barack Obama have held a joint press conference in which they urged UK voters to stay in Europe.
Introduced by the prime minister as "My friend Barack" the US President said, "The UK is at its best when its helping to lead a strong Europe."
"Let me be clear, he began. "This is something that the British voters have to decide this for themselves but ... speaking honestly, the outcome of that decision is a matter of deep interest to the US because it affects our prospects as well.
The prime minister said Britain's "collective power and reach" is "amplified by the EU" and now is the time to "stick with allies" in Europe and around the world."
Mr Cameron said that he and the President have discussed how Nato can contribute to efforts to deal with Europe's migrant crisis, and stressed that the EU has never constrained the UK's special relationship with the US.
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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