Deputy Prime Minister Nick Clegg today said he "massively regrets" being unable to deliver on his pledge to prevent university tuition fee increases.
But he issued a plea to students taking part in today's protests to look at the details of the Government's proposals, which he insisted were fairer than either the existing regime or the graduate tax backed by the National Union of Students.
Mr Clegg - whose Liberal Democrats have become a focus for student anger over tuition fees - came under flak from callers when he appeared on BBC Radio 2's Jeremy Vine Show while the protests took place.
One lifelong Lib Dem voter, Bill Price of Seaton, Devon, told Mr Clegg he had "sold the Liberal Party for your own personal ambition" by going into coalition with Conservatives.
But Mr Clegg insisted that leaving the Tories to form a minority government would have been a recipe for instability and would probably have meant a second general election by now.
Challenged over why he was now backing tuition fee increases which he vowed to oppose during the election campaign, Mr Clegg said: "Of course I massively regret finding myself in this situation."
But he said that the fact the Liberal Democrats did not win the election outright, and that the country's finances were worse than they had anticipated, meant that they had to accept "compromise" on issues like university funding.
Asked how it felt to see pictures of students hanging him in effigy, Mr Clegg said: "I'm developing a thick skin."
He added: "I regret of course that I can't keep the promise that I made because - just as in life - sometimes you are not fully in control of all the things you need to deliver those pledges.
"But I nonetheless think that when people look at the detail of these proposals (they will) realise that all graduates will be paying less per month than they do at the moment and the poorest quarter will be paying much, much less and we will be making it easier for some of the youngsters currently discouraged from going to university to go to university.
"I hope that over time - perhaps not overnight - people will say 'OK, this was controversial, it was difficult for the Liberal Democrats, but actually they have put something into place which will finally allow our education system to do something which it hasn't done for generations, and that is to promote rather than thwart mobility."
It was a "scandal" that the two private schools Westminster and Eton - which he and Prime Minister David Cameron attended - provided more undergraduates at Oxford and Cambridge than the 80,000-strong of disadvantaged pupils receiving free school meals, said Mr Clegg.
Under the new system, he said that students would pay nothing towards their fees until they have graduated and are earning £25,000, when their contribution would amount to less than £7 a week.
Meanwhile the top 30% of graduate earners will pay "over the odds, so that they provide a subsidy to poorer students". Over their working lives, richer graduates are likely to pay back more than double the fees they have incurred, he said.