The United Kingdom will exit the European Union after 43 years, as a majority of British voters chose "leave" over "remain" in the country's EU referendum on Thursday.

Tallies showed a 52 percent to 48 percent split for leaving. The result plunged Britain into an uncertain future, with both serious and immediate political and economic ramifications.

The "leave" campaign took a surprising early lead hours after the polls closed, and edged further ahead as the counting continued. With "leave" leading, the British pound plummeted to the lowest in 31 years, the The Associated Press reported. U.S. financial markets were expected to open sharply lower.

"The dawn is breaking on an independent United Kingdom," said Nigel Farage, the anti-EU leader of the U.K. Independence Party. "Let June 23 go down in our history as our independence day."

The U.K. will now enter into a two-year negotiation with the EU on how its exit will be handled, although it could take even longer. The country is bracing for market turmoil and a potential large scale depreciation in the value of the British pound.

Prime Minister David Cameron, who urged Britons to “remain," announced he would resign later this year following the vote.

Neil Hall / Reuters Ballot papers wait to be used at a polling station for the Referendum on the European Union in north London on June 23, 2016.

Polls Extremely Tight Before The Vote

Polling in the week before the referendum showed an almost dead heat between the "leave" and "remain" camps, with the latter taking a slight lead Thursday. A notable number of voters -- around 10 percent -- were undecided just days before the vote.

Earlier this month, a number of polls showed the "leave" side pulling ahead after lagging behind for much of the contest. As the referendum neared, however, support for both sides was almost equal.

Meanwhile, Britain's betting houses split with the polls and odds suggested the U.K. would opt to stay in the Union.

Christopher Furlong/Getty Images British Prime Minister David Cameron addresses students and pro-EU "Vote Remain" supporters during his final campaign speech.

The Road To Brexit

Cameron called the EU referendum in late February to make good on a promise from the 2015 election campaign, during which he vowed the country would hold a vote on whether to exit the EU if he were re-elected.

The move was intended to quell the rising influence of the anti-EU, anti-immigration UK Independence Party, or UKIP -- as well as dissenting voices from within Cameron's own Conservative Party.

Some of the roots of Brexit go back much earlier, however. While the U.K. joined the EU in 1973, it never really adhered to the ideological goal of a "closer union" between states. Membership in the Union was initially controversial and prompted a referendum in 1975, which the "remain" side won with 67 percent of the vote.

Though Britain stayed in the Union, it opted out of key EU initiatives such as the move to establish a common currency and the Schengen Agreement allowing border-free travel.

Dylan Martinez / Reuters Nigel Farage, the leader of the United Kingdom Independence Party, leaves a polling station after voting in the EU referendum.

'Remain' Versus 'Leave'

Since Cameron officially set the June 23 date for the vote four months ago, British politics has been embroiled in a heated contest between the "remain" and "leave" camps. The debate over the Brexit also reverberated around Europe, as rising anti-EU parties backed the prospect of a U.K. departure while central figures in the Union, including German Chancellor Angela Merkel and European Council President Donald Tusk, warned of dire consequences should the U.K. opt to leave.

Politicians backing "remain," including Cameron and leader of the opposition Labour Party Jeremy Corbyn, campaigned to convince voters that the U.K. would be far better off as a member of the EU. They largely focused on the economic benefits of staying in the world's largest trading bloc and warned of the potential market turmoil if the country left.

Prior to announcing the referendum date, Cameron also negotiated a deal with the EU to give Britain more autonomy within the organization if it voted to remain. The terms of the agreement give Britain greater control over a range of issues, including social welfare benefits, immigration and political integration with the EU.

Eric Vidal/Reuters British students hold the UK Union and European Union flags in front of the European Parliament in Brussels.

On the "leave" side, former London Mayor Boris Johnson and UKIP's Farage fronted a campaign that focused heavily on immigration and gaining sovereignty from the bureaucracy of the EU. Buoyed by the refugee and eurozone crises in recent years, this anti-EU sentiment resonated among many Britons.

"Leave" campaigners attempted to capitalize on people's frustrations with the Union, and sometimes played to populist nationalist sentiment. These appeals were often controversial, and proponents of "remain" accused anti-EU individuals of fear mongering and xenophobia. Farage drew condemnation from both sides after debuting a poster showing a group of refugees walking through Europe to imply that Britain had lost control of its borders.

In the darkest moment ahead of the vote, pro-EU Labour Party MP Jo Cox was murdered on the street in the town of Birstall by a man prosecutors say yelled nationalist slogans including "keep Britain independent." The attacker, Thomas Mair, may have had ties to right-wing extremist groups and family members said he had a history of mental illness. Both the "remain" and "leave" sides suspended their campaigns for days after the attack, amid concern that the Brexit debate's rhetoric had become toxic.