Tens of thousands of people are expected to march through London in protest against the government's austerity measures.
Labour leader Ed Miliband is among dozens of speakers due to address crowds in Hyde Park - where the march ends.
Demonstrators want the coalition to end public service cuts and instead create policies they say can create growth.
The government says austerity measures are vital to cutting the deficit.
Union leaders recently criticised Labour for supporting a public sector pay freeze.
The Trades Union Congress (TUC), said workers and campaigners from across the UK would be joining the demonstration.
Earlier this month, Mr Cameron warned more "painful decisions" would be necessary to repair the UK economy, adding that he would not waver from austerity measures.
'Wages falling'However, TUC general secretary Brendan Barber said: "The evidence is mounting that austerity is failing.
"More than 2.5m people are out of work, a further three million are not working enough hours to make ends meet, and wages have been falling every month for the last three years."
Organisers are hoping Saturday's march will repeat a mass demonstration in 2011 over controversial pension reforms, which was attended by more than 250,000 people.
Mark Serwotka, leader of the Public and Commercial Services (PCS) union, is expected to say: "Almost everyone now agrees that austerity isn't working and that this government's policies are making our economic situation worse, not better."
Calls for a mass walk-out over spending cuts have grown in recent months, with the TUC Congress voting in September to look into the practicalities of organising a general strike.
A government spokesman "It is disappointing that some unions insist on pushing for irresponsible and futile strike action which benefits no-one. As we have said time and again, pension talks will not be reopened and nothing further will be achieved through strike action."
Conservative Party Chairman Grant Shapps said of Ed Miliband: "You can't be serious about clearing the deficit when you attend a march that calls for an end to austerity."
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