David Cameron has resisted pressure to back down on tax credit cuts as tens of thousands of anti-austerity protesters marched outside the annual Conservative Party conference in Manchester.
There will be no review of the cuts in next month's Autumn Statement, the Prime Minister said, brushing off calls from within his own party for a rethink of the changes announced in George Osborne's Budget.
The independent Institute for Fiscal Studies has warned millions of people will lose up to £1,300 a year from the Budget changes, which were condemned by trade union Unison as a "cruel tax credits snatch-and-grab".
Conservative former minister David Willetts urged Mr Cameron and Mr Osborne to "ease" the policy in the Autumn Statement, telling The Times: "There is a real risk that it could turn sour as some of those hard-working families that politicians love realise they are heavy losers."
Mr Cameron said the introduction of the £7.20-an-hour "national living wage" and continued increases to the personal tax allowance would protect the poorest.
Before Tories entered office in 2010, the tax credit bill had spiralled from £6 billion to £30 billion, with nine families out of 10 entitled to receive the benefit, he said.
He told BBC1's Andrew Marr Show: "We are moving to an economy where you get paid more and where you pay less in tax, rather than paying more in tax and getting the money back in tax credits. That is a better system.
"As a country, we have had to make difficult decisions in order to get rid of what was the biggest budget deficit almost anywhere in the world.
"Of course, if you don't tackle excessive welfare and make reductions there, you have to either put up people's taxes or cut the NHS or cut education, which I don't want to do."
Asked directly whether the changes could be reviewed, Mr Cameron said: "No, we think the changes we have put forward are right and they come with higher pay and lower taxes."
He insisted a family with someone earning the minimum wage would be "better off" overall as a result of changes made by the government to tax thresholds, benefits, tax credits and the minimum wage.
Anger over tax credits featured strongly on banners waved by protesters in bright sunshine outside the Manchester Central conference centre, which was guarded by a heavy police presence.
Other demonstrators displayed placards opposing war in Syria and restrictions on trade union strike activities.
Several costumes and banners featured images of pigs following claims in a new biography that Mr Cameron took part in a bizarre initiation ceremony at an Oxford student society.
Questions about the Tory leadership succession loomed over the annual gathering - the first since Mr Cameron made a surprise announcement he will stand down by the election scheduled for 2020.
Chancellor George Osborne was forced to fend off suggestions he was lining himself up as Mr Cameron's successor after giving a wide-ranging interview to the Mail on Sunday in which he discussed his courtship of wife Frances, his "irregular Anglican" religious beliefs and his unlikely fondness for gansta rap group NWA.
The Sunday Times reported that as many 18 ministers and former ministers - including Theresa May, Sajid Javid, Jeremy Hunt, Owen Paterson and Liam Fox - were considering mounting a bid for the leadership, though Mr Javid immediately denied he was planning to throw his hat into the ring.
Mr Cameron confirmed his plans to quit before the 2020 election, saying he thought "10 years is enough". But he insisted he remained "passionate" about being prime minister, telling Andrew Marr: "I leap out of bed every morning."
He said the choice of his successor would not be a "coronation" and it was "great" that a number of "stars" were being considered as potential leaders.
With the conference taking place with Conservatives the sole party of government for the first time in 19 years, Mr Cameron cautioned against "complacency or back-slapping".
He insisted the party would not drift to the right in response to the selection of left-winger Jeremy Corbyn as Labour leader.
The Prime Minister told Marr: "As others are losing their heads and lurching off, we will be absolutely in the common ground, delivering for the working people of Britain."
Asked whether he now had an opportunity to "kill off the Labour Party as a serious national force", Mr Cameron said: "We are not in control of what happens to the Labour Party.
"It seems they have - as I put it - run off to the hills. We are in control of what we do.
"There'll be no complacency or back-slapping here this week - maybe a little bit of mild celebration of the election. The main theme is going to be about delivering people the things that we promised."
Labour's shadow minister without portfolio Jon Ashworth said: "David Cameron has shown that his promises to stand up for working families are a complete farce.
"He ruled out any changes to his government's cuts to tax credits, which will make working families on average £1,300 worse off next April.
"His claims that these families will be compensated by his so-called national living wage have already been rubbished by the Institute for Fiscal Studies."
Unison general secretary Dave Prentis said: "Millions of low to middle income working families will be the victims of the Chancellor's cruel tax credits snatch-and-grab next year.
"Ministers are punishing millions of working people who just want to provide for their children.
"The Government must think again, admit tax credit cuts were a huge mistake, before millions of families suffer yet more pain under austerity."