EDINBURGH, Scotland — The fate of the United Kingdom was at stake Thursday as Scotland began voting in a referendum on becoming an independent state, deciding whether to unravel a marriage that helped build 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 in 15 hours on Thursday, the future of the 307-year-old union with England hangs in the balance.

More than 2,600 polling places opened Thursday at 7 a.m. (0600GMT, 2 a.m. EDT) and will close at 10 p.m. (2100GMT, 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.

Polls suggest the result is too close to call, with the pro-independence Yes side gaining momentum in the final weeks of the campaign.

First Minister Alex Salmond was casting 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.

Pro-independence forces got a last-minute boost from tennis star Andy Murray, who abandoned neutrality and 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.

On a foggy morning in Scotland’s capital, Edinburgh, a heavy stream of voters began arriving at a polling station in the city center the moment it opened. The mood was electric, tinged with nervousness.

One of the first, Anne Seaton, said she had voted Yes — “because why not?”

“Scotland got under the English Parliament by mischance” in 1707, she said. “It’s time now for Scotland to make a deliberate decision for independence.”

Voters expressed a mixture of excitement and 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 of the votes. The result is anticipated Friday morning.

Roberts said he was looking forward to watching the results 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.

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

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