David Cameron's election campaign has "descended into desperation", Nicola Sturgeon claimed, as she told voters they have "48 hours to get the Tories out".

The SNP leader criticised the Prime Minister as he warned the public they risked "five long years" of a minority Labour government reliant on "bribes" to smaller parties like the SNP if the Conservatives were not handed a clear mandate at the ballot box.

Warning of the potential "chaos" of such a set-up, Mr Cameron is telling voters: "Backroom deals. Bribes. Ransom notes. Chaos. Not just for the week after the election. But for five long years.

"It doesn't bear thinking of. Our defences weakened. Our ability to pay our way questioned. Our United Kingdom threatened."

But Ms Sturgeon said the SNP showed, following the 2007 Holyrood election, that minority government can work.

She said: "David Cameron's campaign has descended into desperation.

"The SNP has demonstrated that minority government can be stable, successful and effective.

"We were a minority government for four years and it was so successful that the voters chose to turn it into a majority government."

The Scottish First Minister made the comments during a visit to a nursery in Livingston, West Lothian.

Setting out her message, she said: "(There are) 48 hours to get the Tories out, to get an alternative to austerity and to make Scotland's voice heard."

She went on: "The fact of the matter is, if there's an anti-Tory majority on Friday morning, I want to see that anti-Tory majority come together to get the Tories out, but then make sure that it's replaced with something better.

"The SNP will be a positive, constructive and progressive force in the House of Commons but will stand up very firmly for the things we believe really matter."

In a speech in Dumfries last night, the First Minister dismissed suggestions that it must be the party which wins the most seats in the Commons that forms the government, insisting it must reflect the whole of the UK if it is to be considered legitimate.

Anything else would be undemocratic and insulting to Scots voters, Ms Sturgeon suggested today.

"My point is that a legitimate government is one that can command a majority in the House of Commons," she said.

"But a government that relies on support from parties from across the UK for its majority, I'm arguing, would surely be a more legitimate and more representative government.

"A legitimate government is one that can command a majority, but if Scotland chooses to vote SNP - and it's still an 'If', I take nothing for granted - then Scotland will be saying that it wants its voice to be heard loudly and clearly in Westminster.

"For Westminster politicians to somehow argue that that is illegitimate, I think, is undemocratic and I think it is insulting to voters in Scotland."

The SNP is ahead of Labour in the polls north of the border, with analysis of the results of one survey suggesting the party could be on course to take all 59 of Scotland's seats.

Speaking on BBC Radio Two's Jeremy Vine Show later, Ms Sturgeon played down the prediction.

"Politicians in elections usually try to talk up their chances," she said.

"The SNP is not going to win every seat in Scotland."

The First Minister said that even if such a result was delivered, it would not be a mandate for another independence referendum.

"If there was ever to be a mandate for another referendum, it would have to be obtained in a Scottish Parliament election," she said.

"In order for the SNP to put forward a proposal in our manifesto for the Scottish Parliament election, something clearly would have to change from the circumstances that prevailed in the referendum last September."