The largest public sector workers' union has called on its 1.3m members to join resistance to government cuts.
Unison general secretary Dave Prentis said the government "won't know what hit them" if services, pay or pensions are cut.
David Cameron said the coalition would try to protect the vulnerable.
Meanwhile, a BBC survey revealed many councils had already earmarked billions of pounds worth of savings, with social care services likely to be badly hit.
The government says its first priority is reducing the UK's £156bn debt but Labour has said spending cuts this year could derail the economic recovery.
England's councils must save £1.1bn as the government tries to cut the UK's deficit by £5.7bn this financial year.
Scotland, Wales and Northern Ireland have been asked to find savings totalling £704m.
Many councils are waiting for the emergency Budget on 22 June before they fully reveal their plans.
The prime minister told the BBC the government had some difficult decisions to make but the country was in a "financial mess" and action had to be taken.
He said: "We will try to do our best to take the whole country with us and to protect the poorest and the most vulnerable but we're iin a mess financially and we've got to sort it out.
"There's certainly no bashing of the public sector.
"We should revere people who work in the public sector, who work in our hospitals and our schools, and who do a great job."
But he said it was time to take another look at public sector pensions.
The BBC's home editor Mark Easton said: "Local government won't know exactly how deep the axe has struck until the autumn, but council leaders are planning for the worst.
Local government leaders say they are doing what they must do, but unions claim the plans amount to a vicious attack on services and jobs.
A number of services across the UK are under threat, including plans to upgrade Oxford Railway Station with a £10m new walkway and platform. Elsewhere:
- Up to 200 Blackpool Council staff who help unemployed people are set to lose their own jobs.
- A campaign is under way to ensure a £44m ferry link from Cornwall to the Isles of Scilly still goes ahead.
- In Leeds, the council says the cuts would have an effect on all services, including schools and support for families living in deprived areas.
- North Yorkshire councils say transport schemes and education budgets are going to be hit the hardest by the cuts.
- Leicestershire County Council warns of job cuts and threats to its childcare, teenage pregnancy advice and vulnerable people's services.
- More than 20 county council offices could close across Somerset as the authority tries to reduce its £400m debt.
Mr Prentis told the BBC: "We're planning for a big fight, a big campaign that will last four years.
"And we believe the people who make these cuts - many of them on an ideological basis - the people who make these cuts, we will make them pay a political price."
Brendan Barber, general secretary of the Trades Union Congress (TUC), said the cuts would cause more longer-term problems and would not reduce the deficit but make it worse.
But Secretary of State for Communities and Local Government Eric Pickles said "every single penny" had to be accounted for.
He told the BBC: "There is still excessive waste to cut back on. Our plans to throw open the council books and publish all spending over £500 online will root out wild overspends, expensive mistakes and waste and could be key to saving the public purse millions of pounds."
"Local authorities have to play their part but we have protected the £29bn that local government receives every year and slashed red tape, giving them the tools they need to protect the front line."
Richard Kemp from the Local Government Association, said he wanted Deputy Prime Minister Nick Clegg to guarantee that council workers' pensions would be excluded from any cutbacks.
He said: "We have a funded pension system and we've been showing restraint on public sector pay, which is more than central government did."
Their comments came the same day it was revealed that the number of unemployed in the UK had risen to 2.47 million people.
During prime minister's questions, Mr Cameron said everyone in the UK would have to make sacrifices.
"If we don't do something about it by the end of this Parliament we will be paying £70bn in debt interest.
"That is more than we spend on schools and more than we spend on defence. It would be a tragic waste of money."