After two years of campaigning, pollsters say that the outcome of the vote on whether to bring the 300-year old union to an end is too close to call. Most polls show the pro-U.K. "no" campaign narrowly in the lead despite a late surge by the pro-independence "yes" camp—but pollsters say thousands of Scots won't make up their minds until they are standing in the polling booth.
As polls opened at 7 a.m. U.K. time (2 a.m. EDT) a trickle of voters made their way to Edinburgh City Council offices to cast their ballot on a misty, damp morning.
Among the first to vote were Elvira Sanderson, 62, and Annette Miller, 48, both of whom work at a nearby rest home for ex-servicemen.
"It's important because you are voting for your country," said Ms. Sanderson. A U.K. law prohibits news organizations from publishing accounts of how individuals voted in the referendum while the polls are open.
In Glasgow, Scotland's largest city, people waited outside the Garnethill Multicultural Community Centre to cast their votes. The atmosphere was friendly and cheerful as people arrived on foot and bicycle.
"Change is going to happen, whatever happens," said Ben Dalgety, 21, who works at a local soccer ground in the city.
Pollster YouGov released a survey Thursday morning that put pro-union support narrowly in the lead. The survey showed 49% of the more than 3,000 people polled between Monday and Wednesday supported keeping Scotland in the union and 45% supported independence. The rest were undecided or didn't know, YouGov said.
Turnout is forecast to be high, with voting officials expecting more than four million Scots to vote at 2,600 locations across the country by the time polling closes at 10 p.m. U.K. time (5 p.m. EDT). The result of the referendum is expected to be announced around 6 a.m. or 7 a.m. local time Friday.
The vote caps a long and intense campaign that reached a fever pitch in its final months. U.K. Prime Minister David Cameron and other senior politicians have spent the past two weeks in frantic dashes back and forth to Scotland from England to shore up support for the union and persuade Scots that independence is fraught with risks.
In an effort to woo voters eager for greater autonomy but wary of quitting the U.K., London has pledged to transfer more power over tax and spending to Scotland's semiautonomous parliament in Edinburgh.
Alex Salmond, leader of the Scottish National Party and the central figure in the diverse, grassroots-led pro-independence "yes" campaign, has urged Scots to take the plunge, saying the nation will be richer and happier if it breaks away from the U.K.
Famous Scots have also expressed their views on the referendum, with tennis player Andy Murray the latest to do so in a message on Twitter. "Huge day for Scotland today! no campaign negativity last few days totally swayed my view on it. excited to see the outcome. lets do this!" he tweeted.
Young people will play a larger role than usual in the vote, with everyone aged 16 or over eligible to vote in the referendum, even though the age requirement is usually 18 years in general elections.
Aidan McMillan, 16, said he voted on his walk to school in Glasgow, shortly before 9 a.m. "I've been decided since it started, in 2012, when the campaign started," he said, noting he was just 14 at the time. Mr. McMillan, wearing the moss-green blazer of his school, said he wasn't voting the same way as his family, or most of his friends.
"We've got quite into the campaign, for yes and no," said Paul Feeney, 17, who attends the same school as Mr. McMillan. "I tried to stay undecided for a while, and then just today sort of decided on it," he added.
The referendum has electrified Scotland and brought to the surface strong emotions on both sides of the debate.
Anne Law, 46, walked away from a polling station at a primary school in Glasgow with tears in her eyes.
"I feel quite emotional now," Ms. Law said, adding that she was "just excited. As soon as the date was announced, really, we've been talking about it."
She then drove away, with a Scottish flag flying from the roof of her car.