The Deputy Prime Minister, who admitted he had benefited from a privileged upbringing, insisted it was time to break down barriers preventing poorer children reaching their potential.
"It just is not right that for too many young people, birth has become destiny, that the circumstances of someone's birth should shape, narrow and limit opportunities at school, at college, at university, the labour market - and more than that, on some evidence, limit the length of time you will live," he said.
As part of a social mobility strategy published today, he called on companies to allocate internships through open competition rather than being allocated to the "well-connected".
But he faced accusations of hypocrisy over his own intern experience at a Finnish bank, which was set up through a friend of his financier father.
Labour MP John Mann said: "It is total hypocrisy and really desperate for him to attack internships now.
"His policies are holding down social mobility in this country but he enjoyed all the advantages of family connections himself."
Campaign group Intern Aware also claimed that Mr Clegg's Liberal Democrats were among the "worst offenders" for unpaid internships.
Co-director Ben Lyons said: "It is encouraging that politicians have finally woken up to the scandal of Britain's unpaid intern culture.
"But as part of any solution, Nick Clegg must address the widespread use of unpaid interns in his own party."
Mr Clegg said the Lib Dems would be putting their own internship system "on a much more transparent footing" for youngsters, including making applications "name and school blind".
Civil service internships are also to be advertised on a central website from 2012, ending informal placements within Whitehall.
Mr Clegg called for financial support for interns, covering out-of-pocket expenses or even offering a wage, amid concerns that many youngsters cannot afford to undertake unpaid positions, particularly in London.
"For too long, internships have been the almost exclusive preserve of the sharp-elbowed and the well-connected," Mr Clegg said.
"Unfair, informal internships can rig the market in favour of those who already have opportunities.
"We want a fair job market based on merit, not networks. It should be about what you know, not who you know.
"A country that is socially mobile bases opportunity on your ability and drive, not on who your father's friends are."
Prime Minister David Cameron called on MPs to recruit interns "from backgrounds who wouldn't always get those opportunities".
The Government will be tracking progress against its social mobility goals with the publication of a set of "indicators".
Ministers are also setting up a Social Mobility and Child Poverty Commission, to be chaired at first by former Labour minister Alan Milburn.
The new commission will be independent and staffed by "a small staff of experts", producing annual progress reports to Parliament.
The social mobility strategy highlights several "critical moments" in terms of people making the best of their lives.
Indicators to be studied and compared across social groups include birth weights, school readiness at age five, educational achievement and entry to the "most selective" third of universities and economic activity between 18 and 24.
They will be incorporated into the business plans of Whitehall departments, forcing them to consider the impact of policies on social mobility.
Mr Clegg said: "A society which does not have social mobility, is a more segregated society, a more divided society, a more frightened society and physically - a more gated society."
Nick Pearce, director of centre-left think-tank the IPPR, said the report card indicators were "mostly a sub-set of those used in the Labour years".
But he backed Mr Clegg's focus on internships, saying: "They are a source of cheap labour for employers, who can rely on better-off families supporting their children to get a vital first step on the jobs ladder.
"Interns should be paid a proper wage and openly recruited on merit."
Kitty Ussher, former Labour minister and director of the Demos think-tank, said the Government was right to highlight early intervention projects but said the strategy failed to address the impact of spending cuts on social mobility.
"If councils cut youth services, as well as non-essential services for children and the elderly, this will reduce the earnings potential of informal carers, many of whom are already finding it hard to juggle work and family commitments," she said.
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