EDINBURGH, Scotland — Scots held the fate of the United Kingdom in their hands Thursday as they voted in a referendum on becoming an independent state, deciding whether to unravel a marriage with England that built an empire but has increasingly been felt by many Scots as stifling and one-sided.

The question on the ballot paper is simplicity itself: “Should Scotland be an independent country?” Yet it has divided Scots during months of campaigning, and polls suggest the result is too close to call.

A final Ipsos MORI poll released Thursday put support for the No side at 53 percent and Yes at 47 percent. The phone survey of 991 people has a margin of error of plus or minus three percentage points.

Until recently, polls suggested as many as one in five voters was undecided, but that number has shrunk dramatically. In the latest poll, only 4 percent remained uncertain how they would vote.

Voters lined up outside some polling stations even before they opened at 7 a.m., and on the fog-shrouded streets of Scotland’s capital, Edinburgh, there was a quiet thrum of excitement at history unfolding — an electric mood tinged with nervousness.

The future of the 307-year-old union with England will be decided in 15 hours on Thursday; polls close at 10 p.m. (2100 GMT, 5 p.m. EDT). Turnout is expected to be high, with more than 4.2 million people registered to vote — 97 percent of those eligible. Citizens and residents as young as 16 can vote.

A Yes vote would trigger 18 months of negotiations between Scottish leaders and London-based politicians on how the two countries would separate their institutions ahead of Scotland’s planned Independence Day of March 24, 2016.

Many questions — the currency independent Scotland would use, its status within the European Union and NATO, the fate of Britain’s nuclear-armed submarines, based at a Scottish port — remain uncertain or disputed after months of campaigning.

After weeks in which British media have talked of little else, the television airwaves were almost a referendum-free zone Thursday. Electoral rules forbid discussion and analysis of elections on television while the polls are open.

On the streets it was a different story, with rival Yes and No billboards and campaigners outside many polling places.

The campaign has generated an unprecedented volume and intensity of public debate and participation. The Yes side, in particular, has energized young people and previously disillusioned working-class voters.

For some voters, this was a day they had dreamed of for decades.

“Fifty years I fought for this,” said 83-year-old Isabelle Smith, a Yes supporter in Edinburgh’s maritime district of Newhaven, a former fishing port. “And we are going to win. I can feel it in my bones.”

For Smith, who went to the polling station decked out in a blue-and-white pro-independence shirt and rosette, statehood for Scotland was a dream nurtured during three decades living in the United States with her late husband.

“The one thing America has that the Scots don’t have is confidence,” said Smith, who returned to Scotland years ago. “But they’re getting it, they’re walking tall.

“No matter what, Scotland will never, ever be the same again.”

Smith’s three children and seven grandchildren are all Americans, and several flew to Scotland for the referendum to support her.

Many opponents of independence agreed that the campaign had reinvigorated Scottish democracy.

“I support the No side, but it’s been a fascinating, worthwhile discussion about Scotland’s future,” said writing consultant David Clarke.

“If it’s a No, it’s a win-win situation. If it’s a Yes, we will have to deal with the fact that it’s a Yes.”

First Minister Alex Salmond, leader of the independence campaign, cast his vote near his home in northeastern Scotland. If the Yes side prevails, he will realize a long-held dream of leading his country to independence after an alliance with England formed in 1707.

In a final speech on Wednesday night, Salmond told voters: “This is our opportunity of a lifetime and we must seize it with both hands.”

Pro-independence forces got a last-minute boost from tennis star Andy Murray, who signaled his support of the Yes campaign in a tweet to his 2.7 million followers early Thursday.

Anti-independence leaders, including former Prime Minister Gordon Brown, have implored Scots not to break their links with the rest of the United Kingdom, and have stressed the economic uncertainties independence would bring.

At Edinburgh polling stations, excitement vied with apprehension about Scotland’s choice.

Thomas Roberts said he had voted Yes because he felt optimistic about its future as an independent country.

“Why not roll the dice for once?” he said.

Once the polls close, ballot boxes will be transported to 32 regional centers for counting. The result is anticipated Friday morning.

Roberts said he was looking forward to learning of the outcome in a pub, many of which are staying open overnight.

“I’m going to sit with a beer in my hand watching the results coming in,” Roberts said.

Many Yes supporters planned to stay up late in bars, or to gather in symbolic spots like Calton Hill, overlooking Edinburgh — hoping the sun will rise Friday on a new dawn and not a hangover.

But Financial consultant Michael MacPhee, a No voter, said he would observe the returns coming in “with anxiety.”

Scottish independence was “the daftest idea I’ve ever heard,” he said.