Senior Labour figures have attacked the coalition government's plans for early cuts, accusing it of "broken promises".
Leadership hopeful David Miliband accused Lib Dem Deputy PM Nick Clegg of "hypocrisy" for backing £6.2bn of cuts, after campaigning against them.
PM David Cameron told the Sunday Times action had to be taken to tackle the "huge amount of debt" but pledged to explain "the purpose behind the pain".
Mr Clegg told the Observer the cuts would not mean a "repeat of the 1980s".
The Tory-Lib Dem coalition government is due to announce further measures to tackle the £156bn deficit, in an emergency budget on 22 June.
Mr Cameron told the Sunday Times there was a "serious problem" with Labour's forecast of 3% growth in 2011.
He said: "There is a huge amount of debt that has got to be dealt with. Crossing our fingers, waiting for growth and hoping it will go away is simply not an answer."
He said the government would try to take people with them "on this difficult journey" and would explain to people "the purpose behind the pain."
There has been much speculation the government will put up VAT to try to address the £156bn deficit.
Asked if he could rule it out, Mr Cameron said: "We want expenditure to bear the burden of what needs to be done."
He added: "Sometimes politicians haven't tried hard enough to reduce inefficient spending and have reached for taxes too quickly. We shouldn't do that."
He said "massive welfare bills", public sector pay and "the bureaucracy that has built up over the past decade" needed to be addressed.
'Very old politics'
In an interview with the Observer his deputy, Mr Clegg said: "It is important that people understand that fiscal retrenchment does not mean a repeat of the 1980s. We're going to do this differently."
He said there was an assumption on the centre-left that austerity measures were regressive and right wing but argued some of the biggest cuts had been made by centre-left governments in Sweden, the US and Canada in the 1990s.
Ministers say cuts must be made quickly to show Britain is serious about cutting the deficit. But they were attacked by senior Labour figures running for the party leadership.
David Miliband told BBC One's Andrew Marr Show: "I think there's breathtaking hypocrisy in the interview of Nick Clegg in the Observer today saying he's going to have progressive cuts, cuts that he argued against while he was campaigning."
And he said Mr Cameron had promised only waste would be cut this year - and yet had announced the end of Labour's Future Jobs Fund - which guaranteed work or training to 18-24 year olds out of work for six months.
"That is not waste that is being cut. The danger is we are wasting lives and we are repeating the mistakes of the 1980s and I think it's very important that while we have a Labour leadership election, we don't forget we have to be a fighting opposition as well.
"It's broken promises that we are seeing, not broken Britain that was talked about in the election campaign. It's very old politics that is being done by the coalition."
His leadership rival Ed Balls also told the BBC the Lib Dems had broken promises, made before the election and during talks with the Labour Party about the possibility of setting up a coalition with them, on topics including education spending.
"The Liberals... fought their election on a manifesto which said they would not cut spending now and it now seems they are going to do that it seems... They are even breaking the promises they made after the election, let alone before."
Labour campaigned against making cuts in spending this financial year, arguing it could jeopardise the fragile economic recovery.
But several European countries have since announced austerity measures - including Italy, Spain and Portugal. Mr Balls told the programme: "I think what's happening in Europe is a really really dangerous and big mistake."
"I think the idea that the German finance ministry's prescription for smaller countries, which is radical and big spending cuts now, is the right prescription is out of step with world opinion."