Early figures on Friday indicated the turnout for Scotland's historic independence referendum could be the highest ever in the United Kingdom, for a vote that could break up the centuries-old union.

A survey released after polling stations closed said independence could be narrowly rejected, but both "Yes" and "No" campaigners said it was too close to call as counting was underway in Scotland's 32 voting districts.

"So that's that. Polls have closed. What an amazing, emotional, inspirational day of democracy this has been. Now we wait," said Nicola Sturgeon, deputy leader of the Scottish National Party (SNP) that demanded the vote.

Crowds gathered for all-night parties in Glasgow and Edinburgh, draped in the blue-and-white Saltire flags of Scotland and setting off flares, while campaigners waited anxiously for results in counting centres.

"We are going to stay out till the result," said Dylan McDonald, 17, one of Scotland's 16- and 17-year-olds who have been able to vote in the referendum for the first time.

Turnout in many voting districts was touching 90 percent, meaning turnout could break the previous record set in the general election of 1950.

The historic decision gripped many Scots who previously took little interest in politics, igniting passions and raising the prospect of deep changes to the governance of the union no matter the result.

Prime Minister David Cameron promised greater powers for Scotland's parliament in a last-minute bid to convince voters to stay in the union, prompting politicians in his Conservative party to call for the same treatment for England.

Newspapers of Northern England united to demand the devolution of powers to their regions in a "fair deal after so many promises were made to Scotland during the referendum campaign".

Cameron will speak on the future of the United Kingdom as soon as the referendum outcome is issued, and reports say if independence is rejected he will announce plans to change the division of power in the high-centralised union.

'Vote yes... for us'

Some ballot boxes were brought by helicopter and others by boat from remote islands to be counted after polls closed, with the final result predicted to arrive in the early hours of Friday.

The closure of the airport on the Isle of Lewis due to fog meant ballot boxes would have to travel by slower fishing boat.

At the counting centre in Scotland's oil city Aberdeen, boxes of postal votes were tipped out onto tables at the stroke of 10:00 pm when polls closed, and officials immediately began sorting the ballots.

Election officials in Glasgow said they had contacted police over a handful of allegations that people had turned up to vote only to find their names already crossed off the ballot sheet.

The question for voters at Scotland's more than 5,000 polling stations was "Should Scotland be an independent country?" and they are asked to mark either "Yes" or "No".

International media descended on the Edinburgh venue where the city's ballots will be totted up to witness a count that could have repercussions from Spain to Canada.

The SNP has said it hopes for full independence by 2016 if it wins, and a range of separatist movements sent representatives to Scotland to learn from the election.

"Scots, please, vote yes, for yourselves, but also for us," Daniel Turp from the Parti Quebecois said at a press conference in Edinburgh where 29 European separatist movements also signed a declaration calling for self-determination.

Leaders of France and Spain warned that separatism risked undermining Europe in the run-up to the vote.

A palace spokesman told Sky news Queen Elizabeth II was following events from her family home Balmoral Castle in rural Scotland.

She is "kept abreast of information... from her team of advisers in London and Edinburgh," the spokesman said.

'Ripped out of the UK'

Many people in the rest of the United Kingdom are concerned about the prospect of Scottish independence, which would sever a deep bond and cut the UK's surface area by a third.

"At last the threat we have over Scotland's future may be lifted if people vote the right way," said pensioner Alistair Eastern, 60.

"We just have to hope that it turns out with the right result and Scotland isn't ripped out of the United Kingdom by the nationalists."

A "Yes" vote would not mean independence overnight but would trigger complex talks on how to separate two intertwined economies and eventually end a union dating back to 1707.

A lot of the debate has focussed on the economy, what currency an independent Scotland would use and whether it could be a member of the European Union.

Scotland's Parliament, opened in 1999, holds some powers devolved from Westminster to set policy in certain areas of domestic policy, such as health and education.

For many voters, this was not enough.

"I voted 'Yes' because I decided Scotland should be governed by itself," said university administrator Sarah Rowell, 36, in Edinburgh.