People from across the country have started a march in London in protest at the coalition government's spending cuts.
The Trades Union Congress predicts more than 100,000 people will join the march, to be policed by 4,500 police.
The TUC said it was deploying more than 1,000 stewards to ensure the event remained "family friendly".
Ministers say the cuts are necessary to fix the public finances and critics must come up with an alternative.
More than 600 coaches have been taking people to London on Saturday morning, with marchers assembling from 1100 GMT on Victoria Embankment and Lower Thames Street.
They will then walk to Hyde Park for a rally from 1330 GMT where speakers will include Labour leader Ed Miliband.
Unison general secretary Dave Prentis has joined marchers gathering on Victoria Embankment.
Continue reading the main story
Our alternative is to concentrate on economic growth through tax fairness”
Unite general secretary
"It's going to be absolutely enormous, incredible, far bigger than we expected," he said.
It's going to be an absolutely incredible demonstration of ordinary working people, and ordinary families, saying this coaliton has got to stop cutting jobs and public services."
TUC general secretary Brendan Barber said those taking part in the March for the Alternative will include community groups, pensioners and public sector workers.
He said they were urging the government to spend more public money - not less - on projects to create jobs and boost the economy, and to crack down on tax evasion and avoidance in order to claw back more for the Treasury.