Thousands of people around the country have protested against the so-called "bedroom tax", which will cut the benefit of social housing tenants with a spare room.
Campaigners, who have gathered in more than 50 towns, said the move targets the most vulnerable in society, including carers and the disabled.
The tax is expected to affect around 660,000 households when it comes into effect next month, with an average loss of £14-a-week, according to the charity Crisis.
David Orr, chief executive of the National Housing Federation described the bedroom tax as "bad policy, bad economics and bad politics," adding: "These are huge changes that affect the lives of millions and that you know are just wrong. The bedroom tax is one of these once in a generation decisions that is wrong in every respect.
"It’s bad policy, it’s bad economics, it’s bad for hundreds of thousands of ordinary people whose lives will be made difficult for no benefit – and I think it’s about to become profoundly bad politics. It is incompetent and unfair and should be repealed."
Iain Duncan Smith said earlier this week that the bedroom tax was needed because the current system was “unfair on taxpayers… unfair on those in over-crowded accommodation and unfair that one group of housing benefit tenants cannot have spare bedrooms and another group are subsidised".
Protesters on the demonstrations, who were heard chanting "axe, axe the bedroom tax", were joined by Labour, SNP and MSP politicians, as well as trade unions and Occupy London. As many as 90 politicians have already signed an early day motion against the reforms.
Earlier on Saturday, hundreds of protesters gathered in Trafalgar Square in London before marching to Downing Street. Protesters chanted: "David Cameron is a w**ker, is a w**ker, is a w**ker". Many lining the streets wore Guy Fawkes masks.
Mass gatherings were also held in Edinburgh and Glasgow, with many dressed up in anti-bedroom tax T-shirts. One demonstrator was dressed up as death, holding a sign that said: "Some taxes should not be as certain as me."
Prior to the protests, an anti-bedroom tax song was uploaded onto YouTube, which has gained more than 90,000 views so far. The lyrics to the chorus are: "Oh ye cannae have a spare room in a pokey council flat/ Iain Duncan Smith and Co have put an end tae that/ They say 'live in a smaller house', they say that is their plan/ When the odds against you finding one are ninety-nine to one."
The national organiser of the protests, Dr Èoin Clarke, said on Saturday: "This is a cruel policy that primarily hits single parents, and the adult disabled.
"Even children deemed disabled but not 'severely' so, are affected.
"Carers, the terminally ill, battered wives and husbands are all affected.
"There are times in history when people must stand together in defence of common decency - that time has come."
Iain Duncan Smith confirmed that foster carers and members of the Armed Forces will not be subjected to the tax, which the Coalition has insisted is a "spare-room subsidy". However, campaigners say the U-turn has not gone far enough.
According to the National Housing Federation (NHF) those affected also include separated parents who share care of their children and couples who use a spare bedroom when recovering from an illness or operation.
The government's poverty tsar, Frank Field, has condemned the "bedroom tax", calling it “a form of social engineering that would have made Joseph Stalin proud” in a recent interview with The Guardian.
Writer John Wight explained why the cuts had galvanised so many campaigners in a blog for the Huffington Post UK:
"A home, whether bought or rented, represents more than just shelter in our lives, and even more so in the case of the poorest and most vulnerable in society.
"A home represents a history, where children are brought up, parents pass away, in which good and bad times are shared. It is essential to a sense of being and self worth, not to mention dignity. These things, without which no decent existence is possible, are precisely the things that are under attack with this bedroom tax.
"This is why the sheer and utter cruelty of it transcends words such an iniquitous or unfair. It is nothing short of a violation of the human rights of those impacted, compounded by the fact that it will have a disproportionate impact on the disabled and elderly and sick. The stress being suffered by its victims leading up to its implementation will already have been immeasurable, leaving them feeling even more vulnerable and isolated in the face of decisions being made affecting their lives in which they have no input whatever."