PRIME Minister David Cameron has admitted pro-union parties were having to play catch-up with the SNP's independence referendum campaign and needed to "galvanise" opinion across the country.

However, Mr Cameron said he believed a "whole number of Scottish parties and politicians" would join the campaign.

He added: "My voice is just one of those voices. You will see Labour politicians, Liberal Democrat politicians and people from all walks of life, and many people who don't like politics at all, who might even despise all politics and politicians, but who will come forward and defend the United Kingdom."

Speaking before his meeting with First Minister Alex Salmond, he said: "Do we need to do more? Yes. Do we need those of us who care about the United Kingdom to work together? Yes.

"Do we need to galvanise opinion across the country? Yes. But is that opinion there ready to be galvanised? Absolutely.

"I'm quite convinced the argument both of head and heart for the United Kingdom is so strong that when we make that appeal we can make this campaign."

During his Edinburgh speech, Mr Cameron said he hoped there would be contributions from senior Labour figures.

Mr Darling welcomed what he called a "marked change of tone" from the Prime Minister.

He said: "This was not the traditional speech saying that we will all lose out. This was a much more measured speech."

He added he and the Prime Minister had "huge" differences of opinion over many issues, but said they could work together.

He said: "It is grown-up politics to co-operate with people where you agree with them – as I'm sure we will see Alex Salmond on a platform with Tommy Sheridan."

Mr Darling also defended the decision not to set out what extra powers Scotland could control if it rejects independence.

He said: "It took 20 years to work out the devolution settlement. If we vote to stay in the UK, I hope we can look at the issue a bit faster than that. But this is a difference of tone and I welcome that."

Mr Cameron also met Labour leader Johann Lamont and Liberal Democrat leader Willie Rennie, as well as Conservative colleague Ruth Davidson.

Ms Lamont said: "The discussion about Scotland's future cannot be a fight between two things Scotland rejects – separation and the Tories." Ms Davidson said Mr Cameron had made a "passionate and powerful case".

Two anti-cuts protesters were arrested over public order offences outside the hotel where Mr Cameron made his speech. He used a side door at the Scottish Government's St Andrew's House headquarters, where he met Mr Salmond, to avoid demonstrators.

Meanwhile, the devo-max option should not be offered to Scottish voters alone, a committee of peers has said.

Devo-max, which could create a different tax regime between Scotland and England, should be put to all UK electors because it affects the whole population, the House of Lords constitution committee said. The peers also called for a straight "yes or no" question.

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l To read the full text of David Cameron's speech yesterday, and Alex Salmond's address in London on Wednesday, go to www.heraldscotland.com/ politics