Parents in households where one of them earns above £50,000 a year lost some or all of their right to child benefit at midnight despite a long campaign against the move.
They will lose all their child benefit if either partner is paid £60,000 or more, and part of it if one is in the £50,000–£60,000 pay band.
Interviewed on the BBC's Andrew Marr programme, Mr Cameron defended the policy which will save the Government £2bn a year.
"You cannot deal with the deficit just by taking more in tax from the very richest – although we are. Nor can you deal with the deficit just by combating welfare, fraud and evasion and excessive welfare at the bottom," he said. Mr Cameron added: "So taking away child benefit from people earning over £60,000 – that's only the top 15 per cent of the country – I'm not saying those people are rich, but you know I think it's right that they make a contribution."
Tory party chairman, Grant Shapps told the BBC's World This Weekend: "I speak as one of the 15 per cent of parents... I have three children, I have just filled in the form… I feel the pain."