Between 400,000 and 500,000 teachers, nurses, firefighters, council and NHS workers, other public sector employees, students, pensioners and campaign groups from across the UK marched through central London to a rally where union officials and Labour leader Ed Miliband condemned the "brutal" cuts in jobs and services.
Violence flared away from the rally when a group of hundreds of activists, not connected with the union protest, clashed with police. They set off fireworks, threw paint and attacked shops in Oxford Street, Regent Street and Piccadilly.
Topshop and HSBC had their windows smashed, while paint and glass bottles were thrown at a Royal Bank of Scotland branch.
Covering their faces with scarves, they fought with police and disrupted traffic, throwing lightbulbs filled with ammonia at officers and lighting a fire.
Nine arrests were made and some police officers were injured.
UK Uncut, an anti-cuts direct action group, later occupied the Fortnum & Mason store in Piccadilly, claiming the firm had "dodged" paying taxes.
TUC general secretary Brendan Barber said he "bitterly regretted" the violence, adding that he hoped it would not detract from the massive anti-cuts protest.
He told the Press Association: "I don't think the activities of a few hundred people should take the focus away from the hundreds of thousands of people who have sent a powerful message to the Government today.
"Ministers should now seriously reconsider their whole strategy after today's demonstration. This has been Middle Britain speaking."
Mr Barber said unions would now step up pressure on the Government, especially MPs in their constituencies, and launch a series of protests next week in defence of the NHS."
London Ambulance Service said 30 people were treated for injuries throughout the day, 11 of whom were taken to hospital, ranging from assault to collapsing with illness.
Commander Bob Broadhurst said: "The main march has gone very well. Their estimates are 250,000 - maybe more - have come to central London and protested peacefully. That has gone as we expected.
"Unfortunately, we have had over 500 criminals effectively attacking premises in the Oxford Circus area, causing damage.
"We anticipated there would be some problems. We have minimised the damage caused. We'll never have enough officers to protect every building in central London. it cannot be done.
"The actual march has gone according to plan. Those damaging buildings have had nothing to do with the TUC."
Mr Miliband, who did not take part in the march, told protesters at the mass rally in Hyde Park that he was proud to stand with them, adding that the Government was wrong to make such deep cuts in public services.
He was heckled by a small number of protesters when he said that "some cuts" were needed, but most people applauded his speech.
"The Tories said I shouldn't come to speak here today but I am proud to stand with you. People are here from all walks of life and different backgrounds, speaking for mainstream Britain.
"Our struggle is to fight to preserve, protect and defend the best of the services we cherish because they represent the best of the country we love.
"We know what the Government will say: that this is a march of the minority. They are so wrong. David Cameron: you wanted to create the Big Society - this is the Big Society.
"The Big Society united against what your government is doing to our country. We stand today not as the minority, but as the voice of the mainstream majority in this country.
"There is a need for difficult choices, and some cuts. But this government is going too far and too fast and destroying the fabric of our communities."
Len McCluskey, general secretary of Unite, told the protesters they were bearing witness to services closing, old people going without care, libraries, swimming pools and parks going to "ruin" and young people heading for a life on the dole.
"But you represent a spirit of resistance in every workplace and community that says we are not going to have our way of life killed so that the rich and greedy can live as they please."
Mr McCluskey said that every time Deputy Prime Minister Nick Clegg stepped out of doors it cost police £2 million to protect him, adding: "We cannot afford that any more - if you were to go on a national tour we'd be bankrupt."
The Unite leader attacked the Government's "assault" on the NHS, warning ministers that privatising the health service would spark the same protests as those against the poll tax when Margaret Thatcher was Prime Minister.
He also urged Labour MPs to hold the Government to account rather than simply waiting for the next general election.
Dave Prentis, general secretary of Unison, said the turnout was "absolutely enormous" and showed the anger of ordinary working people at the Government's cuts".
"We always expected a huge turnout because Unison alone has laid on 500 coaches and a number of special trains but the numbers are simply incredible.
"These are ordinary families and working people, many with their children to send a strong message to David Cameron to halt the damaging cuts which are leading to the loss of tens of thousands of jobs and the closure of services including libraries and care homes."
Demonstrators started arriving in London hours before the march was due to begin, turning the Embankment into a sea of colour with banners, balloons and entertainers filling the banks of the Thames.
Steel bands, choirs, performers and dancers practised at the head of the march as tens of thousands of people, many with their children in tow, blew horns and whistles waiting patiently to march through central London to Hyde Park.
Shadow chancellor Ed Balls was heckled by some protesters when he turned up to speak to union leaders at the head of the march this morning.
He said Labour was determined to continue campaigning against the spending cuts and would create jobs and build houses if they got back into power, but several people shouted out at him "what are you going to do for us Ed?"
Education Secretary Michael Gove acknowledged the public concerns about the planned cuts but insisted that the Government would not be deflected from its strategy.
"Of course people will feel a sense of disquiet, in some cases anger, at what they see happening, but the difficulty we have as the Government inheriting a terrible economic mess, is that we have to take steps to bring the public finances back into balance," he told the BBC Radio 4 Today programme.
He said that there were "really big dangers" for Mr Miliband in addressing the rally at the end of the march.
"One is that people will say 'You are calling for a plan B from the Government, you don't even have a plan A.' More than that, you are associating yourself with a march which could, I'm afraid, move from being a family event into being something darker," he said.
A spokesman for civil liberties group Liberty said: "There can be no doubt that the official trade union-led demonstration was overwhelmingly civil, peaceful and good-natured and that the policing response was generally proportionate.
"However the demonstration appeared to have been infiltrated by violent elements who periodically separated from the main route in order to attack high profile commercial properties and the police before melting into the demonstration once more.
"This minority presented significant challenges for the police and trade union stewards alike and at times jeopardised both the safety and ability to protest of those with peaceful intent."
A Scotland Yard spokesman said five officers were injured during the protests.
Four were treated for minor injuries and one was taken to hospital with a groin injury.
Labour leader Ed Miliband tonight condemned the violence which erupted away from the march, saying: "Hundreds of thousands of people peacefully protested today.
"They are the true voice of today's march.
"I unequivocally condemn those who have committed acts of violence. There is no excuse for it. It is unlawful and wrong."
Treasury Minister Justine Greening was adamant tonight that the Government would not change course as a result of the protests.
"We do have to get on with tackling the financial problems we have been handed by the Labour Party," she said. "We are going to stick with the course that we have set."
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