Union leaders will call for a general strike as tens of thousands of workers, anti-war campaigners, politicians, community groups and other activists stage a huge protest rally against the Government's austerity measures.
More than 250 coaches have been booked to bring people to London, while similar protests will also be held in Belfast and Glasgow.
Labour leader Ed Miliband will be among dozens of speakers at a rally in Hyde Park, joining union officials who will spell out the impact of spending cuts on public services.
There have been growing calls for the TUC to organise a general strike following last year's day of industrial action by public sector workers over the Government's controversial pension reforms.
Saturday's events are aimed at calling on the coalition to end its public sector spending cuts and other unpopular policies.
Bob Crow, general secretary of the Rail, Maritime and Transport (RMT) union, will say: "The marches are a building block towards the objective of co-ordinated action and a general strike. That is why RMT says, march today, strike tomorrow."
Public and Commercial Services (PCS) union leader Mark Serwotka will call for co-ordinated strikes to be held as soon as possible after the protest.
He will add: "Almost everyone now agrees that austerity isn't working and that this Government's policies are making our economic situation worse, not better. We're not just marching against the cuts, we're marching for the alternative. An alternative where we invest in our economy and our public services to create jobs, instead of cutting them, where we don't demonise and punish people for being on benefits, we get them back into work and paying their taxes.
"We should be demonising the real scroungers, the ones who really live a something-for-nothing lifestyle, the tax dodgers and the millionaires in the Cabinet who inherited their money instead of working for it like the rest of us."
The TUC Congress last month called for the practicalities of organising a general strike to be considered, which the PCS union said was an "absolute necessity" given the scale of the spending cuts.