Iain Duncan Smith dismissed claims he was “slashing” welfare and insisted he could live on £53 a week.
The Work and Pensions Secretary said he was making the system fairer and giving people the chance to “break free” of benefits.
Ministers launched a fightback as 660,000 social housing tenants deemed to have a spare room began to lose an average £14 a week in what critics have dubbed a “bedroom tax”.
It is part of a package of significant welfare and tax changes coming into force this month, which opponents warn will hit poor families and the disabled particularly hard.
Changes to council tax benefit will see bills for an estimated 2.4 million households rise an average £138 a year, with two million paying for the first time, an anti-poverty group said.
The system has been handed to town halls and operated from Monday, but with 10% less funding.
On Saturday, working-age benefits and tax credits will be cut in real terms with the first of three years of maximum 1% rises well below the present rate of inflation.
Two days later, disability living allowance (DLA) begins to be replaced by the personal independence payment (PIP), which charities say will remove support from many people in real need.
Mr Duncan Smith, whose ministerial salary is equivalent to around £1,600 a week after tax, was asked whether he could live on £53 a week. The former army officer, who married into a wealthy family, replied: “If I had to I would.”
During the course of the day more than 21,000 people signed a petition on the change.org website, calling for Mr Duncan Smith to “prove” he could survive on £53 a week.