Early results from Britain's bitterly contested referendum showed a lead on Friday for supporters of leaving the European Union, contradicting opinion polls and prompting wild swings in the value of the pound.
With results in from the first 13 of 382 voting districts, those in favor of ending Britain's 43-year membership were on 52.4 percent of the vote, while those wanting to stay were on 47.6 percent.
But it was too early to establish a firm trend in a contest that remained too close to call. Opinion surveys pointed to a vote to Remain, and two prominent anti-EU campaigners said they expected to lose.
Nigel Farage, head of the UK Independence Party and a leading voice in favor of leaving the EU, told Sky News as voting ended: "It's been an extraordinary referendum campaign, turnout looks to be exceptionally high and looks like Remain will edge it."
Farage said his prediction was based on "what I know from some of my friends in the financial markets who have done some big polling". Government minister Theresa Villiers, who also campaigned for Britain to leave, told Sky her instinct was that the Remain side had won.
Farage's comments and pro-Remain opinion polls pushed the pound to its highest level this year, above $1.50. But it then plunged after the vote count in the northeastern city of Sunderland showed a stronger-than-expected vote in favor of taking Britain out of the EU.
Sterling fell more than six cents, diving as low as $1.4351 before recovering to around $1.46 in extremely volatile and illiquid trading.
Prime Minister David Cameron had urged Britons to vote Remain, warning that the alternative was a leap in the dark that would hurt trade and investment, bring about a self-inflicted recession, undermine the pound and push up shopping bills and the cost of holidays.
Advocates of going it alone said a 'Brexit' would invigorate the economy by freeing business from suffocating EU bureaucracy, and allow the country to recover its sovereignty and regain control of immigration.
Britain's 27 EU partners are anxiously watching the vote, fearing the departure of the bloc's second biggest economy would weaken Europe's global clout and fuel the rise of eurosceptic movements in other countries.
Ralph Brinkhaus, a senior ally of German Chancellor Angela Merkel, told Reuters: "The released polls show the expected neck-and-neck race. It will remain exciting until the early morning hours. I hope that the British have decided against a Brexit."
POLLS SAY 'REMAIN'
Before a single result had emerged, a survey by pollster YouGov showed Remain ahead by a margin of 52 to 48 percent. Unlike a classic exit poll, it was based on online responses by a pre-selected sample of people rather than a survey of voters as they left polling stations.
Pollster Ipsos-MORI also put Remain in the lead, saying that surveys it had carried out on Wednesday and Thursday gave it a 54-46 margin of victory. An Ipsos-MORI poll published earlier had just a 52-48 split for Remain.
"It's early days and there will be twists and turns through the early hours of this morning but, for now, the markets have taken that YouGov poll as a strong indication that the Remain camp has won," said Jeremy Cook, chief economist at international payments company World First in London.
Professor Vernon Bogdanor, a politics expert who tutored Cameron at Oxford University, told BBC television there was a possibility that Leave would win.
"One can't deny it's a real kick to the British establishment because all three party leaders have favored a Remain vote, business on the whole favored a Remain vote, the financial leaders have favored a Remain vote. The people have not taken their advice," he said.
A vote to leave would send decades of European integration into reverse, marking the first time an independent nation has broken away. It would threaten to open further cracks in a grouping already reeling from successive crises over Greek debt and a mass influx of refugees from Syria and elsewhere.
A vote to stay would leave the EU intact, with its most free-market proponent still a member. However, what began as a domestic political gambit by Cameron has polarized the country and exposed wider challenges facing Europe: public angst over immigration and the falling living standards of many in the world's richest region.
Marred by the murder of a pro-EU UK politician, Jo Cox, who was shot and stabbed in the street a week ago, the campaign and its divisive rhetoric highlighted the populist wave also seeping into the U.S. election race.
A defiant Farage said he was not conceding defeat, even though he feared the result would go against him.
"I hope I am wrong, I hope I am made a fool of, believing that to be the case. Either way, whether I am right or wrong, if we do stay part of this union it is doomed, it is finished anyway. If we fail tonight it will not be us that knocks out the first brick from the wall, it will be someone else," he said.If it votes to stay, Britain has been promised a special status exempting it from any further political integration, but European leaders will still have to address a sharp rise in euroscepticism across the continent.
A Brexit vote, however, would deal a potentially fatal blow to the career of Cameron, who called the referendum and campaigned for the country to stay in, against a Leave camp led by rivals from within his own Conservative Party.
"Thank you everyone who voted to keep Britain stronger, safer and better off in Europe - and to the thousands of Remain campaigners around the UK," Cameron said on Facebook.
In a letter, 84 eurosceptic Conservative lawmakers called on Cameron to remain prime minister regardless of the result. It marked the first attempt to heal the deep rifts that have opened up in the ruling party since the start of the campaign.
The signatories included prominent Leave campaigners Boris Johnson, the former mayor of London, and Michael Gove, a cabinet minister and personal friend of Cameron.
But despite the statement of loyalty, Cameron would face huge pressure from the country at large to step down as prime minister if Britons have defied him and voted to leave.
Results are due to be announced throughout the night.
The vote came on a day when London and parts of southeast England were hit by torrential rain, causing floods and widespread transport chaos.
Five London polling stations opened late as staff struggled to get there, and two closed briefly because of flooding but were re-opened in back-up locations. Local media reported some voters had to wade through water to reach a polling station.
"In London/southeast and want to vote in the #EURef? Make sure you plan now to get back to your local polling station by 10pm!" the Electoral Commission said during the evening on Twitter as commuters struggled with train cancellations.
Among those affected was Johnson, who cast his vote with just 25 minutes to spare after returning to the capital from his daughter's graduation in Scotland.
"Let's see, let's see. It's in the hands of the people now," he said when asked how he felt about the vote.