David Cameron has rejected accusations that his Big Society policy is simply a mask for government spending cuts.
The prime minister said his initiative to give a greater role for community and voluntary groups was intended to change the way the country was run.
Writing in the Observer, he accepted that such organisations might need help in the face of government cuts.
Labour leader Ed Miliband said the Big Society was a "failure" and a return to the Thatcherite policies of the 1980s.
The Big Society is central to Mr Cameron's plan to shrink the size for the state, with individuals and voluntary groups taking more responsibility for delivering public services.
But charity leaders have accused it of being undermined by government spending cuts.
Sir Stephen Bubb, who heads the Association of Chief Executives of Voluntary Organisations, told Sky News: "If you want to build a bigger society you have got to have the foundation there and that's the charities, the social enterprises, the community groups and they are the very ones that are being hit. They are making redundancies, they are cutting the work they do in communities.
"So you can hardly build a bigger society if the very people at the heart of that vision are cutting back on the work they do."
Defending the Big Society in the Observer, Mr Cameron said: "Building a stronger, bigger society is something we should try and do whether spending is going up or down.
"But there is a broader point to be made. As the state spends less and does less - which would be happening whichever party was in government - there would be a positive benefit if some parts of society were to step forward and do more."
The prime minister said billions of pounds of government contracts would be opened up to bids from groups within society.
"The scale of this opportunity dwarfs anything they've ever had before," he said.
Mr Cameron said the government would announce details in the coming week of a £100m transition fund to help groups at a time when local councils were seeing their budgets cut.
'Here to stay'
There will also be a Big Society bank to inject £200m of working capital for projects approved under the scheme - with an announcement due on Monday.
Rejecting the notion that the Big Society was too vague, Mr Cameron said: "True, it doesn't follow some grand plan or central design.
"But that's because the whole approach of building a bigger, stronger, more active society involves something of a revolt against the top down, statist approach of recent years.
"The Big Society is about changing the way our country is run. That's why the Big Society is here to stay."
But Labour leader Ed Miliband branded the Big Society a "failure", which had been destroyed by government cuts.
"No one can volunteer at a library or a Sure Start centre if it's being closed down," he said in an article for The Independent on Sunday.
"And nor can this Conservative-led government build a Big Society while simultaneously undermining its foundations with billions of pounds worth of cuts to the voluntary sector."
He added: "What does all this substance and style remind us of? The early 1980s, and the Thatcher government which appeared intent on ripping apart our social and economic fabric."
And shadow cabinet office minister Tessa Jowell said she was unconvinced by the government's defence of its idea.
"The fact that David Cameron has had to come rowing to the rescue of his version of the Big Society suggests that all the problems that he identified in his Observer newspaper article are true.
"It's too vague, that people don't understand what it means, centrally that you cannot combine greater responsibility for civil society, voluntary and community organisation, with cuts in the money that support them to do the work that they do."
But, in a robust defence of the Big Society on BBC1's Politics Show, Cabinet Office minister Francis Maude rejected the Labour leader's claim that volunteers would have nowhere to work.
He said: "It's such a silly thing to say because actually there is so much going on, there are so many ways in which people can get involved and do get involved.
"We have got to look at ways of doing things differently. The idea that you've got to have your Sure Start centre and your library and everything all in separate places because otherwise there won't be places for people to volunteer is simply absurd.
"It's such an old-fashioned way of looking at things."
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