Initial reactions to Britain's decision to leave the European Union began pouring in Friday from Europe and the wider world, ranging from concern and disappointment, to glee among Eurosceptics.

European Union leaders were reeling Friday after the vote has plunged the EU into deep uncertainty, not least over how to negotiate the unprecedented departure of a member state.

European Council President Donald Tusk told reporters that Britain's 27 partners were "determined to keep our unity." "It is 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 does not kill you makes you stronger'," said EU Tusk. EU leaders should "start a wider reflection on the future of our union," he said. European Commission President Jean-Claude Juncker was hosting talks Friday with the leaders of the European Council and Parliament, along with Dutch Prime Minister Mark Rutte, whose country holds the EU's rotating presidency.

European Parliament President Martin Schulz said the EU assembly will hold an emergency session next week following the U.K.'s decision to leave the bloc.

Britain's vote to leave the European Union is deeply regrettable and marks a watershed moment for European integration, German Chancellor Angela Merkel said on Friday. Merkel told reporters she had invited the leaders of France and Italy as well as the European Council president to Berlin on Monday to discuss how to secure European unity after Britain's referendum vote to quit the 28-member bloc.

Merkel said Germany had a particular interest and responsibility in European unity succeeding.

German Foreign Minister Frank-Walter Steinmeier called the referendum outcome "sobering," adding: "This looks to be a sad day for Europe and for Britain." Germany's Justice Minister Heiko Maas said in a statement, "it is a black Friday for Europe."

Dutch Prime Minister Mark Rutte said on Friday he was disappointed by Britain's decision to leave the EU, but said it should be seen as an incentive to carry out reforms in Brussels. "Firstly it's a disappointing result. It's also a stimulus to reform the EU," Rutte told Dutch media, shortly before leaving for Brussels to meet with the EU's top officials. Swedish Prime Minister also said Brexit is a "wake-up call" for the EU and that the bloc should demonstrate that it can respond to people's expectations.

Poland's Foreign Ministry was also among those who said Brexit should be perceived as a "warning signal" and "wake-up call" for the EU. Britain's decision is a sign of disillusionment that urges the European Union to reform in order to forestall any future divisions, the ministry said. In a statement Friday after Britain's decision was officially announced, the ministry said that the "disillusionment with European integration and declining trust in the EU" can be seen in some other member nations, but they do not question the European project.

Belgian Prime Minister Charles Michel said that beyond "the blow" Britain has dealt the European Union, the referendum result must be seized on to fundamentally rethink the European project. Michel said he wants a special "conclave" of EU leaders as soon as next month to reassess options, considering discontent is spreading even well beyond Britain. He said "we need to keep a cool head and need to see what new way of cooperation would be possible.

The Czech Prime Minister Bohuslav Sobotka called for a "more flexible, less bureaucratic EU." "Europe must be more operational, flexible, less bureaucratic and much more perceptive to the diversity that its member states represent," said Sobotka, the head of a center-left cabinet.

COMPILED FROM WIRES