Firefighters, nurses and teachers marched alongside unemployed youngsters, anti-war activists and other campaigners in a huge protest against the Government's austerity measures.
Tens of thousands of people took part in a noisy demonstration in London, with similar events being held in Glasgow and Belfast.
Union leaders will call for a general strike to be held as soon as possible after the protests.
TUC general secretary Brendan Barber said the massive turnout showed how unpopular the coalition's policies were.
He told the Press Association: "We are sending a very strong message that austerity is simply failing.
"The Government is making life desperately hard for millions of people because of pay cuts for workers, while the rich are given tax cuts."
Mr Barber said the resignation of chief whip Andrew Mitchell and reports of Chancellor George Osborne travelling in a first class train carriage with a standard ticket showed how out of touch the Government was.
He said: "The Chancellor eventually paid for his ticket, but the rest of us are paying the price for his disastrous policies."
Dave Prentis, leader of Unison, said hundreds of thousands of public sector jobs were being lost as a result of government policies.
He said: "We are fighting for a better future. We are not here today for the millionaires - we are here for the millions of people who don't have a voice. We just can't take any more."
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