People from across the country are converging on London for a march 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.
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.
'Mainstream voices'
The largest union involved, Unite, said so many of its members wanted to take part that it could not find enough coaches or trains to ferry them to London.
Unite general secretary Len McCluskey claimed the response showed that the public anger against the spending cuts was now palpable.
Andrew Burgin, secretary for campaign group Coalition of Resistance, said he thought it would be a "massive demonstration".
More than 600 coaches are due to take people to London on Saturday morning, with marchers planning to assemble 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.He said on Friday that "the voices of the mainstream majority" would be making themselves heard.
"I think the government will be making a great mistake if they somehow dismiss all of the people on that march as troublemakers, or just 'the same old people'. They are not," he added.
Conservatives say Mr Miliband has no right to attend because Labour has not put forward a credible economic plan.
There are some concerns about disorder at the event, and a number of groups have been using the internet to call for the occupation of buildings in the West End.
The Metropolitan Police said it planned to station officers at certain sites thought likely be at risk, such as the Treasury and the entrance to Downing Street.
It has also written to businesses asking them to step up their security and to clear away any loose equipment such as ladders and dustbins that could be used as weapons.But police preparations have been criticised by former Met assistant commissioner Andy Hayman.
Writing on the website of the centre-right think tank Policy Exchange, he said there was "strong intelligence" that "extremist groups" were planning to cause trouble, and officers should be "more intrusive and active" to try to prevent it.
But the TUC hit back, insisting that months of planning and close co-operation with the police would ensure the march would be peaceful.
The TUC is urging people not to join feeder marches and to stagger their arrival and departure times.
Both the police and the TUC will be sending information and advice to protesters during the march via Twitter.
'Kettling' concerns
In a report published on Friday, Parliament's Joint Human Rights Committee praised the Met and the TUC for their close liaison.
But it said it was concerned about the possible use of containment - or "kettling" - on peaceful demonstrators, and expressed surprise that neither the police nor the organisers had raised issues around the technique in their planning.
The Met will for the first time allow observers from human rights group Liberty into its control room for the event.
Met Police commander Bob Broadhurst said he hoped for a peaceful demonstration, but added: "We might end up in some form of containment. We would hope we can keep that for as few people as possible and for as little time as possible."
@'BBC'
(GB2011)
