United States president Barack Obama has warned against what he terms a rise in "crude nationalism."
On a visit to Greece, he has been seeking to calm international fears about his soon-to-be successor, Donald Trump.
Rioters in Athens have thrown Molotov cocktails at police, angry at a visit by United States president Barack Obama.
An estimated 7,000 protesters have taken to the streets to protest the US stance on the war in Syria and the election of Donald Trump as the next US president, among other things.
Greek prime minister Alexis Tsipras has acknowledged he shares the concerns about the President-Elect.
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"I believe that, even if somebody would like to change rapidly the foreign affairs of a country like the US, it would be very difficult. But even if some of us in Europe are afraid it might happen, what we have to do now is build bridges and not walls."
It is not just the Greek prime minister expressing his worry.
German chancellor Angela Merkel has offered this thinly veiled warning to countries wanting to adopt protectionist measures.
"Globalisation is a reality, and we can shape it in such a way that we strengthen multilateral instruments."
In the aftermath of Mr Trump's election, there are serious concerns about the future of the Iran nuclear deal.
French president Francois Hollande says he is hopeful Mr Trump will not follow through on his pre-election threat to tear it up.
"This agreement gives us all the security clues to allow the evaluation of the Iran nuclear agreement. Could the United States, with Donald Trump, challenge this agreement? I don't think so."
Mr Obama insists, during the transition, he will give Mr Trump his best ideas about how to move the country forward.
"I still don't feel responsible for what the President-Elect says or does, but I do feel a responsibility as the president of the United States to make sure that I facilitate a good transition and I present to him, as well as to the American people, my best thinking, my best ideas, about how you move the country forward."
Mr Obama leaves Greece on Thursday to fly to Germany for talks with Ms Merkel.
