"Damn! A bad day for Europe." - Sigmar Gabriel, Germany's vice chancellor and economy minister, on Twitter.
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"I will do everything I can as prime minister to steady the ship over the coming weeks and months, but I do not think it would be right for me to try to be the captain that steers the country to its next destination." - British Prime Minister David Cameron speaking to reporters outside his Downing Street office. He said a new prime minister should be in place by October.
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"It's true that the past years have been the most difficult ones in the history of our Union. But I always remember what my father used to tell me: What doesn't kill you makes you stronger." - EU President Donald Tusk, speaking to reporters in Brussels.
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"The European Union is strong enough to give the right answers to today." - German Chancellor Angela Merkel, speaking in Berlin.
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"A victory for freedom. We now need the same referendum in France and in EU nations." - French far-right leader and presidential contender Marine Le Pen.
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"Huge numbers of people in the EU reject the EU's immigration policy, there's big disappointment with the economic policy." - Slovak Prime Minister Robert Fico, whose country is taking over the rotating EU presidency in July, adding that the bloc's key policies need "a fundamental change."
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Britain's decision to leave the EU "is not the end of the world and above all not the end of the European Union." - Czech Prime Minister Bohuslav Sobotka, on Facebook.
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"Now it is our turn. I think the Dutch people must have the chance to express themselves in a referendum," Dutch right-wing politician Geert Wilders.
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"We must do everything to avoid the domino effect, a situation when other member nations also say that they don't want to be in the European Union any longer." - Polish President Andrzej Duda, speaking in Krakow.
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"Something of the European ideal has been lost today following the UK's vote for Brexit. Today the European flag is at half-mast, but sometimes it is necessary to take a step back before taking a step forward." - Pedro Agramunt, president of the Parliamentary Assembly of the Council of Europe, speaking in Strasbourg.
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"The U.K. will remain a strong and committed NATO ally and will continue to play its leading role in our alliance." - NATO Secretary-General Jens Stoltenberg.
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"What matters now is that we keep Europe together." - German Foreign Minister Frank-Walter Steinmeier, speaking in Luxembourg.
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"I can only give a sigh: So it's done. This is bad news for Europe, bad news for Poland. It means instability of the situation in Britain now." - Polish Foreign Minister Witold Waszczykowski.
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"The people of the United Kingdom ... have declared their independence from the European Union, and have voted to reassert control over their own politics, borders and economy. A Trump administration pledges to strengthen our ties with a free and independent Britain." - Donald Trump, the presumptive Republican U.S. presidential nominee, on Facebook.
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"Major economic and political uncertainty will be a fact of life for some considerable time, likely weighing down markedly on business and household confidence and behavior, so dampening corporate investment, employment and consumer spending." - Howard Archer, global economist for IHS Global Insight, cutting estimates for economic growth in Britain in light of the vote.