GLASGOW - British Prime Minister David Cameron urged Scottish people to reject independence on Thursday in a rare speech in Glasgow in which he argued union was a strong economic advantage.
Cameron said the union between Scotland and England was the "greatest merger in history" as he addressed a conference of the Confederation of British Industry (CBI) in Glasgow.
His visit came on the day that 200 company heads signed an open letter to say independence was in Scotland's "best interest" - underlining the polarised views over the issue three weeks from the historic September 18 referendum.
Questions over whether Scotland's economy could do well alone have been at the heart of the campaign. On Wednesday, a rival group of 130 captains of industry said independence would be "bad for business".
Most opinion polls currently suggest the proposal, which would end more than 300 years of union with the rest of the United Kingdom, is set to be rejected.
The "Yes" and "No" camps have traded statistics and accusations on everything from whether Scotland could keep using the pound to what share of Britain's national debt it should take on if it broke away.
Cameron has kept a low profile on the campaign trail due to his centre-right Conservative Party's unpopularity in Scotland.
But he re-entered the fray in his speech, saying the UK was "one of the oldest and most successful single markets in the world" and that Scotland does twice as much trade with the rest of the UK as the rest of the world put together.
"Our single market is one of our union's greatest advantages. If we stay together, Scottish businesses have better opportunities, Scottish consumers have more choice and Scottish people have more secure jobs," Cameron said.
"Why put that great advantage at risk by going into the great unknown?"
"Cameron isn't welcome"
Yet outside the venue for Cameron's speech, about 120 protesters gathered, chanting and waving the Scottish blue and white flag and banners such as "Another Scotland is Possible".
"We're fed-up to be underneath the blood-soaked of the Union Jack and we're not going to stand for it anymore," said Michael Larkin, 34, an illustrator.
"I've got friends and family who go to a food bank every day to get food to feed their kids because the living wage they've got is not enough." Other protesters criticised the austerity policies of the Conservative party, which currently has just one MP out of 59 in Scotland, where the Labour party is far more popular.
"David Cameron isn't welcome in Scotland because Scotland don't vote Tory (Conservative)," said Samuel Cook, 18, a student.
"I want independence because I want Scotland to make its own decisions. I want to get rid of the Tory party because Scotland has a Tory government we do not vote for," he said.
The pro-independence campaign led by First Minister Alex Salmond argues that the British government's economic policies are unfairly skewed in favour of London and southeast England.
Salmond greeted the pro-independence letter from business leaders by declaring industry was "waking up to the opportunities of independence".
The letter, published by The Herald newspaper, said policy set by people "who truly understand and care most" about Scotland, not politicians in London, would be better.
The signatories range from big business leaders such as Brian Souter - chairman of major transport group Stagecoach and a prominent donor to the independence campaign - to the owners of smaller enterprises such as guest house owners.
Think tank Capital Economics added to the debate on Thursday, warning that independence could lead to significant capital flight from Scotland's giant finance industry.
The report said investors were expecting a "No" vote and there would be a "strong market reaction" in case of a "Yes" victory.