Single mums, disabled people and the homeless were among those who camped out in city centres including London, Edinburgh, Cardiff, Belfast, Liverpool, Manchester and Leeds to warn that the coalition is literally throwing people onto the streets.
Liverpool's protest had the full backing of the city council, which provided waste bins and officer support, with organisers banning all alcohol and drug-taking.
Fascist groups were also barred from attending.
Organiser and Merseyside TUC president Alec McFadden told the Star: "We wanted to show the right-wing media that this sleepout was not one big party but about fighting for vulnerable people who are losing their homes."
He backed calls from Liverpool Mayor Joe Anderson and Salford Mayor Ian Stewart for Labour to get tough and pledge to scrap the bedroom tax.
Introduced in April, it slashes housing benefit for those living in social and council housing if they are deemed to have a spare room, regardless of individual circumstances such as disability.
Tenants with one spare bedroom have their housing benefit cut by 14 per cent, while those who the government says have two or more spare rooms suffer a 25 per cent cut.
Protesters braved the rain in Brixton, south London, to set up tents and gazebos for their overnight stay in Windrush Square.
Organiser Grace Lally said she believed the poorest in society were being targeted by the bedroom tax.
She said that 170 people had been affected on Brixton's Loughborough estate, where she lives.
"I think it's vindictive to go after the poorest people in our society and to blame them for the lack of housing," she said.
"You're only penalised for having a so-called spare bedroom if you're unemployed and you're poor."
The sleep-out also brought together the local community in the market town of Darlington, Durham, including those that had direct experience of being homeless.
Local activist Joanne Land called the tax "an iniquitous attack on the most vulnerable people in society" and said it is not cutting welfare spending but increasing it.
Local college lecturer Dominic McArdle said: "Whoever dreamt up the bedroom tax has displayed an almost psychopathic lack of social conscience.
"This bedroom tax is no different to the poll tax and shows how committed the Tories are to inequality."
In Glasgow protesters set up a camp in front of the Kelvingrove Art Gallery and Museum, opposite the office of Scottish Conservatives leader Ruth Davidson.
Scottish Anti-Bedroom Tax Federation's Gail Morrow said: "The support on the day has been fantastic in Glasgow, with local businesses keeping us refreshed with lunch.
"The passing support has been amazing, with people stopping to talk and show support."