Britain votes: ‘We are leaving.’ World leaders react to Brexit vote.

In a referendum Thursday, British voters opted to leave the European Union, ushering in a period of soul-searching for both the United Kingdom and Europe.

Britain is to leave the European Union.

In a referendum Thursday, the British people delivered a result that defied most expectations, voting by 51.9 percent to 48.1 percent to embrace the "Brexit."

British Prime Minister David Cameron, who had championed the campaign to remain in the EU, made the announcement Friday morning, revealing that he would be stepping down by October.

"We should be proud of the fact that in these islands we trust the people for these big decisions," said Prime Minister Cameron. "We not only have a parliamentary democracy, but on questions about the arrangements for how we're governed there are times when it is right to ask the people themselves and that is what we have done."

Despite inclement weather conditions in parts of the country, voter turnout was its highest for a UK-wide vote since 1992, with 72.2 percent of the electorate registering their preference.

It is the first time in the history of the European Union that a member state has voted to leave, dealing another blow to an institution already groaning under the weight of numerous challenges.

"This is an unprecedented situation but we are united in our response.... The Union of 27 Member States will continue," read a joint statement by Martin Schulz, Donald Tusk, Jean-Claude Juncker, and Mark Rutte, presidents of four EU institutions. "We now expect the United Kingdom government to give effect to this decision of the British people as soon as possible, however painful that process may be."

The future of the United Kingdom itself is now wreathed in uncertainty, with Northern Ireland and Scotland both having opted to remain in the EU, by 55.7 percent and 62 percent respectively. Scotland, which voted against independence in 2014, is now especially likely to push for another opportunity to have their voice heard.

"We said clearly we do not want to leave the EU," said Scotland's first minister, Nicola Sturgeon. "I will do all it takes to ensure these aspirations are realised.... An independence referendum is highly likely."

Many European leaders expressed dismay at the result. German foreign minister Frank-Walter Steinmeier said, "The news from Britain is really sobering;" French foreign minister Jean-Marc Ayrault said that Europe must "rediscover the confidence of its peoples;" and Polish foreign minister Witold Waszczykowski lamented this "bad news for Europe."

But others spoke of the warning it conveyed, saying lessons should be learned. Hungarian Prime Minister Viktor Orban said the EU had failed to answer "major issues such as immigration," while Italian Prime Minister Matteo Renzi said it was a call to change the EU and "make it more human and more just."

Britain will now begin the process of extricating itself from the EU. At some point in the coming months, it will launch a two-year process of negotiation under Article 50 of the Lisbon Treaty, but Mr. Cameron said that would be a task for his successor to initiate.

"I will do everything I can as Prime Minister to steady the ship over the coming weeks and months," said Cameron, "but I do not think it would be right for me to try to be the captain that steers our country to its next destination."

The British pound plunged to its lowest level since 1985, and European stocks tumbled, though both began to recover after the initial shock.

Europe and Britain now enter uncharted territory, and both face the challenge of redefining themselves.

"This is the biggest shock to European politics since the fall of the Berlin Wall," Rob Ford, professor of politics at Manchester University, told Bloomberg.