Voting is well under way on a historic day for Scotland as people determine whether the country should remain part of the United Kingdom.

More than three years after Alex Salmond's SNP secured a landslide victory at Holyrood, the long-awaited referendum on independence is finally taking place.

Polling stations opened at 7am and people have until 10pm to cast their ballot, with the result expected to be known by breakfast time tomorrow.

The question facing voters is a simple one: Should Scotland be an independent country?

The turnout is expected to be high, with 4,285,323 people registered to vote, according to the Electoral Commission, and 16 and 17-year-olds able to take part for the first time.

Scotland's First Minister Alex Salmond was joined by two first-time voters, 18-year-old Natasha McDonald and Lea Pirie, 28, at Ritchie Hall, Strichen, in his Aberdeenshire constituency, this morning.

Mr Salmond gave both women a soft Yes toy as a mascot for their vote and the trio stopped for pictures on their way into the polling station.

Despite long days of campaigning, the First Minister said he managed to get a good rest on the eve of the vote.

"I got a fantastic night's sleep, obviously there's a great deal of anticipation, it's a once-in-a-lifetime opportunity, it's a day that everybody will remember," he said.

"Natasha and Lea are voting for the first time and so will so many people in Scotland, not just 16 and 17-year-olds, but people in their 30s, 40s and 50s.

"We're in the hands of the people of Scotland and there's no safer place to be than in the hands of the Scottish people."

Better Together leader Alistair Darling was greeted by a mixture of cheers and boos as he arrived at the Church Hill Theatre in Edinburgh to cast his vote.

He said: "I'm feeling very confident.

"It's been a long, hard two-and-a-half year campaign, passions have been aroused on both sides, and understandably so because we are talking about the biggest single decision that any of us will ever take in our lifetime.

"But I'm increasingly confident that we will win tonight."

Asked how he slept last night, Mr Darling replied: "I slept very well indeed, thank you."

He added: "This is the biggest decision that any of us in Scotland will ever take. The future of our country is at stake. There is no going back.

"So, I hope as many people as possible will come out and vote but, you know, we are confident."

A group of well-wishers cheered former prime minister Gordon Brown as he made the short trip to his local polling station.

He shook hands with No campaign supporters, as well as one Yes voter, who were waiting for him in the mist at North Queensferry Community Centre in Fife.

Mr Brown said: "Thanks very much for all the help you have given. I don't know if the rain will stay off."

He chatted with supporters for several minutes then posed for photographs before heading into the single-storey community centre.

After casting her vote, Deputy First Minister Nicola Sturgeon tweeted: "I've just voted #Yes to Scotland becoming an independent country. What a wonderful feeling."

The crucial ballot, which could see the 307-year-old union between Scotland and England brought to an end, is expected to go down to the wire, with polls showing the contest is too close to call.

A YouGov survey for The Sun and The Times and a separate poll by Panelbase both found 52% of Scots will to vote to stay in the union, with 48% favouring independence, when undecided voters are excluded.

But research by Ipsos-Mori for the broadcaster STV suggested the gap could be even closer, indicating 51% of people will vote No to 49% saying Yes.

When Scotland's 2,608 polling places close at 10pm the sealed boxes containing the ballot papers will be collected and transported to each local authority's designated count venue - in most cases a school, sports centre or town hall.

The number of ballot papers in each box will be counted by a 5,767-strong counting team and the total will be reported to the chief counting officer (CCO) who will authorise the local counting officer to announce the turnout.

The papers will be sorted into Yes, No and those deemed ''doubtful''. These will need to be judged and possibly rejected as spoiled.

After reporting to the CCO, each local authority will announce its result, with the first declaration expected at around 1.30am and the last at around 6am.

Chief Counting Officer Mary Pitcaithly will announce the will of the nation at the Royal Highland Centre outside Edinburgh tomorrow morning.