Nick Clegg was attacked by Labour MPs in the Commons over his party's U-turn on university tuition fees, as crowds of students protested outside.
Standing in for David Cameron at prime minister's questions, he was bombarded with Labour MPs' questions about plans to raise the annual fees cap to £9,000.
Harriet Harman said he had been "led astray" by Tories who wanted to "shove the cost" of degrees onto students.
The deputy PM said graduates who earned the least will pay less than currently.
The Lib Dem leader came under fire over his own party's about-turn on tuition fees at English universities- as Labour MPs asked him 12 questions on the topic in the half-hour session and he was accused of broken promises and a "shameful" policy.
Before joining the Conservatives in a coalition after the general election, the Lib Dems had pledged to phase out tuition fees altogether over six years. Many of the party's MPs signed a National Union of Students (NUS) pledge not to vote for higher fees if re-elected in May.
But responding to a review of university funding by Lord Browne, the government has announced plans to allow some universities in England to charge up to £9,000 a year from 2012 - the current cap is £3,290.
Ms Harman urged him to go outside, where thousands of protesting students were marching through Westminster, and explain how "fair" the government's plan was.
She also quoted him as having said that increasing tuition fees to £7,000 a year would be "a disaster".
"What word would he use to describe fees of £9,000?," she asked.
As Mr Clegg told her there was a "consensus" across the parties about the need to reform university funding, Ms Harman hit back: "None of us agree with tuition fees of £9,000 a year."
She suggested Mr Clegg had been "led astray" by the Conservatives, who had plans "to shove the cost of higher education on to students and their families".
A series of Labour MPs also got up to press Mr Clegg on the plans - which several of his own MPs, including former leaders Charles Kennedy and Sir Menzies Campbell, have said they will oppose.
But Mr Clegg said the government had come up with a "fair and progressive solution to a very difficult problem".
"The proposals we have put forward will mean that those who earn the least will pay much less than they do at the moment, those who earn the most will pay over the odds to provide a subsidy to allow people from poor backgrounds to go to university," he said.
He said the Lib Dems had to change policy because of the financial situation and compromises made in the coalition agreement.
"This is an extraordinarily difficult issue and I have been entirely open about the fact that we have not been able to deliver the policy that we held in opposition," he said.
He attacked Labour's record on university funding, saying they had opposed tuition fees in 1997 - then introduced them "a few months later" and had commissioned Lord Browne's review which they were now "trashing".
He said Labour's policy to tax graduates was one "half the frontbench doesn't even believe in" and pressed Ms Harman to explain "what on earth her policy is".
Ms Harman said the government's policy was not about reducing the deficit but about "pulling the plug on public funding and dumping the cost on to students".
The previous Labour government introduced tuition fees, but during the Labour leadership campaign, Ed Miliband said a graduate tax was "a fairer alternative", would raise more money and be linked to a graduate's ability to pay.
But the day after Mr Miliband won - and before being made shadow chancellor - Alan Johnson wrote an open letter to him in the Independent, saying: "For goodness' sake, don't pursue a graduate tax. We should be proud of our brave and correct decision to introduce tuition fees."
Shadow Work and Pensions Secretary Douglas Alexander was pressed on whether Labour would push for a graduate tax on BBC 2's Daily Politics.
He said Mr Miliband had said he "wanted to move towards a graduate contribution, that is going to be our approach". "It's right to recognise that it's going to take us time to flesh out how we take forward that approach," he added.
Pressed on whether Labour would adopt a graduate tax policy, he replied: " Yes, Ed said. That was the mandate that he secured from the Labour Party."