World leaders are stunned by Britain's Brexit vote to leave the EU, while fears grow of a domino effect challenging the bloc.

Britons voted 51.9 percent to 48.1 percent in favour of leaving the bloc, upsetting the odds and sending shockwaves around the world as markets and pundits alike got caught out badly by the result.

- German Chancellor Angela Merkel said that "we take note of the British people's decision with regret. There is no doubt that this is a blow to Europe and to the European unification process."

- French President Francois Hollande said that "the British vote poses a grave test for Europe, which must show solidity and strength in its response to the economic and financial risks."

- Italian Prime Minister Matteo Renzi said in a statement at his official residence in Rome "Europe is our house, the house of our children and our grandchildren. We know that the house needs to be renovated, perhaps freshened up, but that it will still be our house tomorrow."

- Prime Minister Justin Trudeau reached out to Britain and the European Union after Britons voted to split from the bloc, saying Canada would "continue to build relationships with both parties."

"The UK and the EU are important strategic partners for Canada with whom we enjoy deep historical ties and common values," the prime minister said in a statement.

"We will continue to build relations with both parties as they forge a new relationship."

- Pope Francis said "It was the will expressed by the people and this requires a great responsibility on the part of all of us to guarantee the good of the people of the United Kingdom, as well as the good and co-existence of the European continent."

- EU Parliament President Martin Schulz hoped the vote would not trigger a domino effect prompting other EU members to quit.

"The chain reaction that is being celebrated everywhere now by eurosceptics won't happen," he said.

- European Union Council President Donald Tusk told reporters in Brussels, "today on behalf of the 27 leaders, I can say that we are determined to keep our unity as 27."

"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 make you stronger," Tusk said.

- US President Barack Obama said that, "the United Kingdom and the European Union will remain indispensable partners of the United States even as they begin negotiating their ongoing relationship to ensure continued stability, security, and prosperity for Europe, Great Britain and Northern Ireland, and the world."

"The people of the United Kingdom have spoken, and we respect their decision," Obama added, in a statement from his White House office.

- UN Secretary General Ban Ki-moon said Friday he expects the European Union to remain a solid partner for the United Nations after Britain's decision to leave the bloc.

"The Secretary-General expects the European Union to continue to be a solid partner for the United Nations on development and humanitarian issues, as well as peace and security, including migration," Ban said in a statement. 

- Russian President Vladimir Putin on Friday said the British government was "overconfident" and "superficial" in the EU referendum, and denied taking a position on Brexit.

"It will have consequences for the United Kingdom, for all of Europe and for us, of course," he added.