Ed Miliband has clashed with David Cameron over proposed cuts to child benefits in his first prime minister's questions as Labour leader.
Mr Miliband said they were not fair and would see families on middle incomes suffering an "enormous" financial loss.
He accused the PM of breaking promises over the universal benefit and urged him to "think again" over the move.
Mr Cameron said his opposite number had only recently "discovered" the interests of middle income families.
In a debut performance cheered on by Labour MPs Mr Miliband said the coalition's plans to cut child benefit for higher rate taxpayers, applying to those on incomes of about £44,000, were a "shambles".
Under the proposals, announced at the Conservative conference last week, he said a couple with three children no longer eligible for benefit would lose £2,500 a year.
"That is an enormous loss the prime minister is inflicting on a particular group of the population," he told MPs.
He asked Mr Cameron to justify how, under the proposals, a family in which one partner worked but the other did not would lose their benefit while families in which both partners worked - and whose combined income might be higher - would retain it.
"It does not strike people as fair. It does not strike me as fair," he said.
Insiting that he was "not defending the rich", he said the prime minister had pledged to protect child benefit during the election campaign and had "no defence" for his subsequent move.
"If he wants to take the public with him on deficit reduction, he has to show his changes are fair and reasonable," he said.
Mr Miliband also chided the prime minister for not giving "straight answers", saying at one point "I might be new to this game but I thought I asked the questions and he answered them".
The prime minister said it was right that the better-off shared the burden in reducing the deficit inherited from Labour and turned on the opposition's economic policies.
"They left behind the biggest deficit in the G20 and he has got absolutely no proposals to deal with it," he said.
Comparing Mr Miliband with predecessor Gordon Brown - "it's not red, it's Brown" - Mr Cameron said the Labour leader was championing the interests of middle income families out of political calculation not conviction.
The first Commons clash between the two men came a week ahead of the spending review, when the government's cuts will be revealed.
One of Ed Miliband's ex-cabinet colleagues said it had been an assured performance.
"I thought that was a very promising start," former culture secretary Ben Bradshaw told the BBC's Daily Politics.
"If he was nervous, he did not show it. I thought the quiet, forensic questioning was quite effective and disarming. David Cameron did not know how to repond."
But former Conservative leader Lord Howard said although Mr Miliband would be able to produce "hard cases" of people worse off as a result of the austerity measures, people needed to ask what the alternative was.
"People have to understand that in the mess we are in, really tough decisions are going to be taken," he told the same programme.
"As long as Labour do not have anything to say about how they would tackle the deficit, really the attacks will not have any credibility."
BBC Political Editor Nick Robinson described Mr Miliband's performance as an impressive debut - "confident, focused, unshowy".
At the start of the 30-minute session, Mr Cameron welcomed his opposite number to his new position and said that he believed there were areas where they could work "constructively together in the national interest".
For his part, Mr Miliband offered his support to the prime minister over elements of the coalition's welfare reform agenda and Afghanistan.
He said he backed the UK's agreement to the attempted rescue mission of British aid worker Linda Norgrove in Afghanistan, during which she was killed.
The PM said a US-UK investigation into her death was continuing after it emerged that she might have been mistakenly killed by US forces during efforts to free her and not by her kidnappers.
Mr Miliband praised Ms Norgrove for her work, saying she had done a "simple job, making the lives of people in Afghanistan better".