Britain's "historic" vote to leave the European Union should not provoke hysterical reactions and the bloc is prepared for Brexit, European Council president Donald Tusk has said.
Mr Tusk said he was fully aware of how dramatic a moment it is and warned that it was impossible to predict what happens next.
He said: "There's no hiding the fact that we wanted a different outcome of yesterday's referendum.
"I am fully aware of how serious or even dramatic this moment is politically. And there is no way of predicting all the political consequences of this event, especially for the UK.
"It is a historic moment but for sure not a moment for hysterical reactions. I want to reassure everyone that we are prepared also for this negative scenario.
"As you will know the EU is not only a fairweather project."
Mr Tusk said the other leaders were "determined to keep our unity as 27" but would have to have a "wider reflection" about the EU's future.
"For all of us the union is the framework of our common future," he said - amid speculation that the UK vote could have a domino effect and prompt similar votes in other countries.
He said he had offered EU leaders an informal meeting in the margins of next Tuesday's European Council summit in Brussels to discuss "the details of further proceedings".
"I will also propose to the leaders that we start a wider reflection on the future of our union."
He acknowledged that recent years had 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."
He said there would be "no legal vacuum" because EU law "will continue to apply to and within the UK " until a formal exit.
"By this I mean rights as well as obligations."