Ed Miliband painted May's general election as a "once in a generation" chance to "rebuild" the country as the election campaign began in earnest with the two main parties trading blows on the state of the public finances.

The Labour leader said his party offered "hope not falsehoods" as he dismissed Tory claims the Opposition had made £20.7 billion in unfunded spending commitments.

In what was billed as the formal launch of a four-month battle to win back power at Westminster at the ballot box in May, he said that working people were on course to end up worse off at the end of a government than before for the first time since the 1920s.

Labour would make cuts to public spending, he said, but deal with the deficit "responsibly" - including through higher taxes on the wealthiest - but had made no promises funded by additional borrowing,

Responding to the threat from Ukip, he insisted that exiting the European Union would be a "dramatic mistake" for the UK and promised "fair rules" on immigration.

"In the next four months, there will be the usual sound and fury," he said.

"But it will all actually come down to something rather simple.

"Who we are. How we want to live together. And how we succeed as a nation.

"This is nothing less than a once in a generation fight about who our country works for.

"It is a choice between a Tory plan where only a few at the top can succeed and our public services are threatened.

"Or a Labour plan that puts working people first, deals with the deficit and protects our NHS."

Chancellor George Osborne will lead senior Tories in an attack on Labour's general election promises, accusing the party of pledging £20.7 billion in unfunded spending commitments.

The Chancellor, flanked by four Cabinet colleagues, will use Treasury costings to claim Mr Miliband's plans for his first year in government do not add up.

But the Labour leader dismissed the Tory analysis as "completely false" and accused David Cameron of "spraying around all kinds of unfunded commitments", including £7 billion of tax cuts.

Deputy Prime Minister Nick Clegg will use his first press conference of the year to brand Tory plans to tackle the deficit "a con" as he continues to attempt to distance the Liberal Democrats from their coalition colleagues.

At the same time, he will warn that Labour's policies represent a "clear and present danger" to the economic recovery as part of his party's strategy of positioning itself as a moderating force on either of Westminster's two main powers in a future coalition.

The Tory analysis of Labour's spending will claim the plans amount to £1,199 more borrowing and debt for every working household.

The BBC's Norman Smith was heckled as he asked whether Labour was "scaremongering" about the state of the NHS in England.

He was called a "pillock" and told to "go back to London" by members of the audience in Salford.

Mr Miliband told the audience "we will hear people with respect" as he attempted to restore order.

He said: "You should talk to people in the NHS. They will genuinely say to you with an edge in their voice 'Where are things going to be in five years' time, what kind of NHS are we going to have?'.

"I met a young doctor a few months ago who had just qualified and he said to me 'You know you have got to have a plan, you know you have got to sort this out because I want the NHS to be there when I'm a doctor'.

"I think there is real fear about the fact that we transformed the NHS in government, it's already gone backwards. If you are proposing as a party to go back to 1930s levels of public spending as a share of national income ... then I think there is real fear about what that means for the NHS and other vital public services."

Mr Miliband indicated that Labour's position on tuition fees would be set out shortly, but he was determined to avoid making the "Nick Clegg mistake" of promising a policy he could not deliver on the issue.

The party has previously said it would reduce fees to £6,000 if it was in office, but Mr Miliband would not say what the party's manifesto commitment will be.

"Watch this space," he said. "I have said before that we will have a proposal on that. What's most important in this area above all others is to make sure that it's a costed and credible plan.

"That's what we are going to do. If I can put it this way, we are not going to make the Nick Clegg mistake. We are not going to make false promises to young people."