A question on civil liberties led to clashes between the four men on DNA retention.

UK Government ministers were forced to review policy south of the border - which allows police to hold profiles of every person arrested indefinitely - after a European Court of Human Rights ruling last year.

However Tories, the SNP and the Lib Dems backed the Scottish system, where the DNA of those accused but not convicted is not routinely kept on the system.

Mr Murphy argued for an extension of that, telling the audience: "It's my responsibility to protect your civil liberties, but also use the best science available to catch some of our vilest criminals across this country."

He added: "We have the ability and the innovation in this DNA databasing to go further. The Labour Party is very clear - we have this ability and we have this know-how and we should deploy it.

"It is our responsibility as a Government to do everything we can to keep the population safe which is why we support this extension on DNA databasing."

But Mr Mundell said the system in Scotland was "much less draconian" and still "works effectively".

He argued: "The Scottish system achieves a much better balance."

Mr Salmond said the system north of the border was "perfectly effective but it doesn't infringe civil liberties by keeping automatically the DNA of innocent people, which is what people are worried about".

And Mr Carmichael said: "I'm not saying we shouldn't have a DNA database, I've never said that, but what we have in Scotland is a meaningful, useful, workable one."

He recalled how he had worked as a deputy procurator fiscal in Aberdeen and added: "There are few things that will annoy police officers and prosecutors more than politicians trying to make cheap politics out of an issue like this."

Closing statements

The debate concluded with each of the four politicians making a direct appeal to voters. Mr Salmond used this to speak out about the "looming squeeze in public expenditure".

He said the three other parties all had an economic agenda which would "cut deep into the fabric of social and economic activity in this country".

The SNP is arguing for the cuts to fall on items such as Trident and the House of Lords and Mr Salmond said: "I believe in cutting the things that don't matter so we can concentrate on preserving the things that do matter.

"I don't believe in cutting deep into public services that make life worth living."

The SNP leader argued economic growth was needed to tackle the budget deficit.

And he said the key issue in the election was "trying to provide an alternative to that miserable perspective of the London-based parties".

Mr Salmond concluded: "The debate between Labour and Tory is a phoney war. The real battle is for Scotland."

But Mr Murphy said the choice facing voters was whether Gordon Brown or David Cameron should be the next prime minister.

He added: "The choice for Scotland is clear as well, because we are at a crossroads. We can either continue away from this recession with Labour or we can go back to the old divisions of the 1980s with the Tory Party."

Mr Murphy said when the Conservatives had been in power at Westminster there had been "15% interest rates, mass unemployment" as well as "dreadful attacks" on the shipbuilding and steel industries and the introduction of the poll tax.

He vowed Labour would create 100,000 new skilled jobs in Scotland.

His message was that there were "big choices" for voters in the election on May 6 and urged them: "Use your vote and go out and vote."

Mr Mundell, however, accused the Scottish Secretary of living in the 1980s and said: "He fights the battles of the past."

But the Tory insisted: "This election is about the future, it's a British General Election and it's about deciding who will be our British Government in London.

"It's about whether we have five more years of the same with Labour or we have change which only the Conservatives can deliver."

He said his party had the policies to deal with the issues that were concerning voters, such as employment and fuel prices, as well as a strategy for economic recovery.

One outcome of the election could be a hung parliament, in which either the Lib Dems or the Nationalists could potentially hold the balance of power.

But Mr Mundell told the audience: "Don't leave the choice to Alex Salmond or to the Liberal Democrats, make the choice yourself. Vote either for the same or vote for change which only the Conservatives can deliver."

Mr Carmichael, however, said the May 6 poll was "exciting because the old parties, Labour and the Conservatives, no longer have it all sewn up".

He argued: "In this election it's not the parties that are in control, it's the people. And if you want change, if you want to do things differently, then you can vote for that."

He accused Labour of having "let Scotland down" adding: "They allowed the banks to trash our economy, they've eroded our freedoms, and they sent our troops to fight and some to die in an illegal war in Iraq."

The Lib Dem said: "They've let you down and they've taken Scotland for granted."

But he added: "You can vote to do things differently, to bring fairness to Britain. That is what the Liberal Democrats offer."

How the debate unfolded:

1. Immigration and taxes

2. The economy

3. Foreign policy

4. Employment

5. Fuel tax

6. Civil liberties and closing statements