Firefighters, nurses and teachers marched alongside unemployed youngsters, anti-war activists and other campaigners in central London, as similar events were held in Glasgow and Belfast.
Brendan Barber, the general secretary of the Trades Union Congress (TUC), said that the strong turnout showed how unpopular the Coalition’s policies were.
He said: “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 Andrew Mitchell, the former chief whip, 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, the leader of Unison, the public service trade union, said that 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.”
More than 250 coaches brought people to London for the demonstration. Protesters blew whistles and held up balloons and banners as they marched through central London.
Ed Miliband, the Labour leader, was expected to 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.
Mr Miliband was expected to tell the rally that whoever was in government now would have to make some cuts.
Groups involved in the protest include the Stop The War Coalition and the Campaign for Nuclear Disarmament, which published adverts highlighting the “disastrous” implications for public services of committing tens of billions of pounds to a new nuclear weapons system.