Thousands of protesters have called for the Government to axe a new "bedroom tax" that will cut benefits for people with a spare room.
Organisers said some 12,000 to 13,000 activists had turned out despite the cold weather in 52 cities across the country.
Under the Government's welfare reforms, those deemed to have one spare bedroom in their council or housing association home face paying £728 extra a year.
The move is expected to affect 660,000 people when it comes into effect next month, although the Coalition this week announced exemptions for approved foster carers and parents with "adult children" serving deployed with the Armed Forces.
The protest's national organiser, Dr Eoin Clarke, said the "cruel plans" would target the most vulnerable in society.
"It punishes the disabled, single parents, carers, the terminally ill, soldiers serving on the frontline 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 the week by Work and Pensions Secretary Iain Duncan Smith, Dr Clarke said: "David Cameron's Government have shown that they just haven't got a clue. 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.
Meanwhile, Scottish Liberal Democrats criticised their own government ministers for failing to stop the tax.
Party members, gathered at their spring conference in Dundee, called on the coalition Government to defer the under-occupancy rules and reconsider its approach. Just one Lib Dem openly backed the government policy at the end of a debate on the emergency motion.