More than 10,000 public sector workers joined a vociferous protest against potential cuts to the "withering" welfare state.
Pensioners, trade unionists, students and other campaigners clogged the streets of central London in bright sunshine to condemn potential cuts after the General Election.
Led by a brass band, crowds of marchers, many brandishing placards, chanted "no ifs, no buts, no public sector cuts."
Frank Cooper, president of the National Pensioners Convention, was among those leading the walk from Temple tube station towards Trafalgar Square.
The 86-year-old, from Barnes, said: "You can see how many people here today are worrying about the welfare state withering away.
"No one is in any sort of doubt about the depth of the financial crisis but whatever the course is to resolve our problems, it is not be those at the sharp end who suffer."
Groups ranging from the TUC to the Disability Alliance and British Medical Association supported the event.
Dr Hamish Meldrum, chairman of the BMA, said the event was apolitical and criticised a lack of transparency over potential cuts.
The GP, based in Bridlington, East Yorkshire, said: "There is a lot of rhetoric in terms of protecting the NHS but we are gravely concerned by any potential cuts after the election.
"This is about showing support and solidarity with other public sector workers."
