Thousands of activists protested in 52 towns across Britain on Saturday demanding the government axe a new "bedroom tax" that will reduce benefits for people with a spare room.
Under the new round of welfare cuts those deemed to have one spare bedroom in their council or housing association home will lose £728 a year.
The coalition cash-grab is expected to affect 660,000 people when it comes into effect next month, although exemptions were announced last week for approved foster carers and parents with "adult children" serving with the armed forces.
Green Benches blogger Eoin Clarke, who helped organise some of the protests, said the "cruel" Con-Dem plans would target the most vulnerable.
"It punishes the disabled, single parents, carers, the terminally ill, soldiers serving on the front line and foster parents with more than one foster child," he said.
"Most Tory supporters are not in favour of this bedroom tax and I have no doubt most are appalled. It is not about left and right - it is about right and wrong."
Commenting on the exemptions announced last week by Work and Pensions Secretary Iain Duncan Smith, Mr Clarke said: "Their U-turns will not work - only dropping the Bill will achieve what we need."
The biggest marches took place in Liverpool and Manchester, with more expected in London, Edinburgh, Glasgow and Cardiff on March 30.
In Scotland around 200 trade union and community campaigners challenged delegates and party leaders at the Scottish Liberal Democrat conference in Dundee to bin the bedroom tax.
STUC general secretary Grahame Smith faced up to Scottish Lib Dem leader Willie Rennie across the metal barrier at the rally outside the conference, telling him the policy was "iniquitous."
Mr Rennie said the tax was "tough but necessary."
Unite's Elaine Dougall warned that the bedroom tax would severely affect women with family caring commitments and called for a massive fightback.
"We're not going to shrink back in to the dark and silence and let them wreck our communities with this attack on social housing," she said.
Protestor Tom Garnett, a mental health worker in Dundee, told the Morning Star: "The people I deal with who suffer problems because of high levels of unemployment and stress are the very people the Lib Dems should be standing up for, not targeting and victimising for the situation they are in."
Another protester, Sinead Daly of Dundee Bin the Bedroom Tax, told the Star that people were aiming to build links with groups across Scotland to take action.
The pressure from the STUC-led lobby appeared to have strengthened the already-existing hostility to the bedroom tax among grass-roots delegates.
In Saturday afternoon's emergency debate on the much-reviled policy, conference voted overwhelmingly against imposing it - a slap in the face for leader Nick Clegg and his Scottish crony Danny Alexander who had visited conference on Friday.
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