George Osborne has defended his £7 billion hit on the welfare budget, insisting that it had been necessary to protect frontline public services.
The Chancellor acknowledged that his package of £81 billion of cuts announced in the spending review had involved "hard choices", but he insisted they were fair.
Labour, however, denounced the Government's "slash and burn" strategy while the Institute for Fiscal Studies said his measures were "regressive", hitting the poor harder than the rich.
Mr Osborne said that he had made a deliberate decision to cut benefits, such as housing benefit for single young people, rather than frontline services.
"I have made a conscious choice. I have decided to try to sustain spending on the National Health Service, on our schools, on some of the important infrastructure like our roads and green energy," he told the BBC Radio 4 Today programme.
Later, David Cameron said higher earners would pay more as a percentage of their income and that fairness was "about asking how much people give as well as how much people get".
"They pay most, not just as an amount of cash, they pay more as a percentage of their income, and that is what the definition of what being progressive is," the Prime Minister said.
None of the cuts would increase child poverty, he said, thanks to extra help for youngsters from deprived backgrounds.
"Fairness is actually about asking how much people give as well as how much people get and I think that we have done it in a way so we can genuinely say: it is difficult, it is tough but it is fair and we are going to take the country with us," Mr Cameron added.
Deputy Prime Minister Nick Clegg accused critics of "frightening" the public by unfairly focusing only on the announcements - such as huge benefit cuts - unveiled in the spending review: "People who are trying to take only one bit of the equation and say 'ah, that shows it is all very unfair' - they are not being very straight with people and frankly they are frightening people and that is not right, frightening people and claiming we are doing unfair things when we are not."