Ed Miliband has branded David Cameron "clueless" during an address at a rally in London's Hyde Park, part of a huge protest against Government austerity measures.
More than 100,000 people took part in the demonstration in the capital, which heard calls for a general strike.
The Labour leader accused the prime pinister of "clinging" to policies which were not working.
He said the coalition was cutting taxes for millionaires and raising them for everyone else.
"It is one rule for those at the top and one rule for everyone else."
Miliband was booed by a small section of a rally in Hyde Park when he said Labour would have to make "hard choices" if it was in Government.
He pledged that if he became Prime Minister he would tax bankers' bonuses, support the building of 100,000 houses and end the privatisation of the NHS.
Bob Crow, leader of the RMT rail union was loudly cheered when he called for a 24-hour general strike.
He also called on Miliband to oppose all spending cuts.
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."
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."
More than 250 coaches brought people to London.
Protesters blew whistles and held up balloons and banners as they marched through central London.
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.
The protesters carried banners which read: 'Cameron Has Butchered Britain', '24 Hour General Strike Now' and 'No Cuts' as they marched through Whitehall towards Hyde Park.
They booed at Downing Street and shouted "pay your taxes" as they passed a Starbucks coffee shop.
Police officers stood outside Starbucks, which has been involved in a row over its tax arrangements.
Some of the marchers shouted "no first class tickets here" in reference to the incident involving the Chancellor.
March organisers said they had been told by police that more than 100,000 people were taking part in the London protest.