Media playback is unsupported on your device Media caption Iain Duncan Smith: "People are subsidised to live in houses where there are spare bedrooms"

Changes to the UK's welfare system are fair, Work and Pensions Secretary Iain Duncan Smith has insisted, as some of the new measures came into effect.

They include cuts to housing benefit for some social housing tenants with a spare room and changes to council tax.

Mr Duncan Smith said it was about restructuring the culture so "people find work always pays".

Some churches, charities and campaign groups, as well as the Labour Party, have criticised the changes as unjust.

The biggest shake-up of the welfare system for decades will also see low income households in many parts of England begin paying council tax for the first time.

'Winners and losers'

Mr Duncan Smith said the government was trying to lower the welfare bill "in a way that we can be as fair as possible - without slashing or attacking people but trying to reform it and change it".

"The reality is that successive governments have come in and when they've had a problem they've cut welfare bills and then later on they've ballooned again," he told BBC Radio 4's Today Programme.

"What I'm trying to do... is to change the process so that we end up restructuring the culture so that people find that work always pays. It doesn't right now."

When asked if he could live on £53 a week, in response to a question posed by a working benefits claimant, Mr Duncan Smith said: "If I had to I would."

Media playback is unsupported on your device Media caption Liam Byrne: "There's nowhere for these people, hit by the bedroom tax, to move to"

Following the interview, a petition hosted by Change.org was set up, challenging the secretary of state to "prove his claim". By 23:00 BST on Monday, the number of signatories had reached more than 90,000.

Market trader David Bennett, 51, said: "Because I've earned the massive amount of £2,700 I have gone over the government threshold for receiving full housing benefit. I was getting £75 a week, I'm now getting I think it is £57 a week and I've also got to pay £5 a week towards council tax which was previously paid for."

Liam Byrne, shadow secretary for work and pensions, told the Today Programme: "Today is a day of big winners and big losers - you've got millionaires who are getting a whopping great tax cut of £100,000 per year and everyone else is taking a hit to tax credits.

"We think that basic strategy is simply unfair."

He said families in Britain were on average £890 a year worse off than they were at the last general election because of the measures and other coalition policies.

From Monday, 660,000 social housing tenants deemed to have a spare room will begin to lose an average of £14 a week in housing benefit.

Pensions Minister Steve Webb said that change was aimed at striking a balance.

"It's bringing people together - some of whom are prepared to trade down so that they don't have the spare room but some of whom are in desperate need and you're not even hearing the voice of those people in this debate," he said.

"These are people - a quarter of a million in Britain, in cramped, overcrowded accommodation - desperate for a family home and there aren't enough homes and we've got to make better use of the houses we've got."

What is happening when? 1 April: Spare room subsidy removed for working-age social housing tenants

1 April: Council tax benefit changed to council tax support and run locally

8 April: Working-age benefits and tax credits uprated by 1% - a below-inflation cap

8 April: Disability living allowance replaced by personal independence payment

15 April: Introduction of cap on total amount of benefit for working-age people

29 April: Universal credit rolled out in test areas Benefit changes explained

Mr Byrne said the benefit change - that his party has dubbed the "bedroom tax" - would take more than £700 a year off some of the poorest people.

He said figures released by Labour suggested 97 per cent of those affected by the change had nowhere else to move to.

"What are those people going to do?" he told BBC Breakfast.

"...[For] many of the people hit by this tax, very often the bedroom is anything but spare... This is not just a cruel punishment, it's incompetent."

Welsh government minister Huw Lewis accused the UK government of "coming for ordinary Welsh people".

On Sunday, four churches joined together to criticise the government's plan as unjust - they fear that society's most vulnerable will be disproportionately hit.

That criticism has come from the Baptist Union of Great Britain, the Methodist and United Reformed Churches, and the Church of Scotland.

Fierce arguments

BBC local government correspondent Mike Sergeant says that while the fiercest argument has been about benefit cuts for social housing tenants with spare bedrooms, some of the other changes will affect many more people.

It has been estimated that two million low income households will pay more, as a result of changes to council tax benefit.

The benefit is being replaced by council tax support and responsibility for it is being moved from central government to councils, but with 10% less funding.

Media playback is unsupported on your device Media caption Iain Duncan Smith said he could survive on £53 a week "if I had to"

Each council has had to decide whether to pass on the reduction to residents - most are to increase bills for low income families.

The Local Government Association, the body that represents councils in England and Wales, argues that if councils had been given more freedom to close the gap in different ways then they would not have to impose bigger bills on the poorest.

Sir Merrick Cockell, chairman of the LGA, said: "The regret is that we agree with the principle that this should be down to local communities - local councils - to decide the right mixture for their area, but we're not [being] given discretion to sort it out ourselves."

Also from this month, most working age benefits will increase by just one per cent - less than the likely rise in the cost of living.

And later this month, an overall limit of £500 a week on claims is beginning in four London boroughs, and will come into force across England, Scotland and Wales over the coming months.

But government plans to test a new universal credit - which combines several benefits into one payment - have been delayed.

Three out of four pilot areas in the north west of England will not now start using the system until July.

The BBC's political correspondent Carole Walker said the overhaul of the welfare system was the government's biggest priority, especially as it attempts to shake off the charge that it is out of touch with those struggling to make ends meet.

Our correspondent added the way in which the changes went down with the public in the coming months would be absolutely critical to the government's reputation.

Note 5 April 2013: This story has been updated to add in the direct quote from Mr Bennett about his financial situation, for the sake of clarity