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* Voting day poll shows unionist support on 54 percent
* Scottish support for union at 52 percent - polls
* Undecideds leave result too close to call
* London offers more powers if Scotland stays
* Results due Friday morning (Adds YouGov poll in second paragraph)
By Guy Faulconbridge and Angus MacSwan
EDINBURGH, Sept 18 (Reuters) - Scotland has voted on whetherto stay within the United Kingdom or declare independence tobreak the 307-year-old union in a finely balanced referendumwith global consequences.
Scotland's verdict on the union should be clear aroundbreakfast time on Friday, but a YouGov poll of 1,828 voterspreviously polled indicated 54 percent of Scots would back theunion while 46 percent would seek independence.
"This YouGov poll indicates the union has prevailed," YouGovresearch manager Laurence Janta-Lipinski told Reuters of thesurvey carried out on Thursday which was not an exit poll. "Itlooks like the union will remain intact for the time being."
Polling booths closed at 2100 GMT from remote highlands andislands to the tough city estates of Glasgow with surveyssuggesting Scots were almost equally divided in a vote watchedclosely by Britain's allies, investors and restive regions athome and abroad.
Recent opinion polls gave the "No" campaign - those infavour of remaining in the United Kingdom - a slight edge. Butas many as 600,000 people undecided up to the last moment heldthe key.
The final poll of polls by the non-partisan "What ScotlandThinks" website, put the No campaign on 52 percent, the Yes campon 48. That measure has never given the independence campaign alead despite a sharp narrowing in recent weeks.
The gap is sufficiently close that if the pollsters have gotit marginally wrong, Scotland could go its own way.
Financial markets lodged a raft of late bets that Scotswould reject independence, pushing the pound and Scottish-basedstocks higher in the final hours of polling.
Analysts say that means there will be a far more dramaticmarket reaction to a "Yes" vote than if Scotland opts to staypart of the United Kingdom.
French President Francois Hollande said the vote would bedecisive for Europe as well as Britain. "After half a century ofbuilding Europe, we risk entering a period of deconstruction,"he said on Thursday.
Those in favour say such fears are overblown. They sayScots, not politicians in London should rule Scotland and see abright future for an independent Scotland in Europe, a fairersociety and strong ties with London.
Those opposed say a split would slow economic growth,diminish the United Kingdom's international standing, threatenthe unity of other countries and tip the balance in favour ofpeople who want Britain to leave the European Union.
Many people see the choice, which has divided families andfriends but also electrified a country of 5.3 million, as one ofhearts versus heads.
Tennis star Andy Murray sent a powerful last-minute message in support of the pro-independence "Yes" vote, tweeting "Let'sdo this".
Alex Salmond, the 59-year-old nationalist leader, toldhundreds of supporters in Perth at a final rally: "This is ouropportunity of a lifetime and we must seize it with both hands."
Salmond has said Queen Elizabeth should stay on as Queen ofScots. She has remained above the fray, in keeping with theconstitution, but said on Sunday she hoped Scots would choose"carefully".
Electoral officials said the result will be announced aroundsunrise on Friday when all regional votes have been submitted.But partial results will give a strong indication after thecount of cities such as Glasgow are declared around 0400 GMT.
With more than 486,000 voters, Glasgow is crucial. Edinburghand Aberdeen, which with Glasgow make up nearly a quarter of theelectorate, are also expected around about that time.
Some currency traders in London prepared to stay up allnight to buy or sell sterling on the result.
WHAT WOULD BE LEFT?
The prospect of breaking up the United Kingdom, the world'ssixth-largest economy and a permanent member of the UnitedNations Security Council, has prompted citizens and allies aliketo question what would be left, while the financiers of the Cityof London have warned of market turmoil.
British politicians, banks and businessmen have closed ranks to warn of economic hardship, job losses and investment flightshould Scots decide to go it alone.
Defence would also be a big question. Britain'ssubmarine-borne nuclear arsenal, part of NATO's defences, isbased in Scotland's Firth of Clyde and the nationalists want itgone.
The United States has made clear it wants the UnitedKingdom, its main ally in Europe, to remain together.
"The UK is an extraordinary partner for America and a forcefor good in an unstable world. I hope it remains strong, robustand united," U.S. President Barack Obama said.
Facing the biggest internal threat to the UK since Irelandbroke away nearly a century ago, Prime Minister David Cameronwill inevitably face calls for his head should he lose Scotland.
Salmond has employed a mix of shrewd calculation andnationalist passion to haul the "Yes" campaign from far behindto within a whisker of winning his dream of an independentScotland.
The 47-year-old Cameron has conceded his privileged Englishbackground and Conservative politics mean he is not the bestperson to win over Scots, although he has made emotional appealsfor Scotland to stay in Britain's "family of nations".
That has left the leadership of the unionist case to theopposition Labour party, the only party with the local supportcapable of checking the secessionist Scottish National Party.
Former Labour Prime Minister Gordon Brown, a Scot who has inrecent days led the battle cry for the union, warned Scots onWednesday that Salmond was "leading us into a trap".
"Say to your friends, for reasons of solidarity, sharing,pride in Scotland, the only answer is vote 'No'," an emotionalBrown thundered, fists clenched, to cheers from unionistsupporters.
MESSY DIVORCE?
In the event of a vote for independence, 18 months have beeninked in to decide the terms of separation but there is noagreement on key issues such as whether Scotland could retainthe pound and how to carve up North Sea oil revenues.
European leaders have warned that an independent Scotlandwould have to apply to rejoin the European Union and could faceresistance. Spain has been especially vocal, fearing it wouldfurther inspire separatists in Catalonia and the Basque Country.
Scotland says it will use the pound after independence, butLondon has ruled out a formal currency union.
Salmond has accused London of orchestrating business leadersto spook Scots after firms from oil giant BP to financialservices group Standard Life cautioned about the risks ofindependence and banks said they would move south.
To blunt Salmond's argument for breaking away, Britain'srulers promised to guarantee Scotland high levels of statefunding and grant Scots greater control over finances.
That pledge has angered Westminster lawmakers.
British leaders accept that whichever way the vote goes, theUnited Kingdom's structure will have to change, as granting morepowers to Scotland will provoke calls for a less centralisedstate from voters in England, Wales and Northern Ireland. (Additional reporting by Alistair Smout in Edinburgh, Writingby Mike Peacock)
