Charlotte Church branded austerity "unethical, unfair and unnecessary" as she joined a quarter of a million demonstrators in a protest against Government cuts.
Families, students and campaigners from all over the country descended on London for the march, which began in the heart of the financial district and snaked its way to Parliament.
Led by a brass band trio, they waved placards, blew whistles and chanted their opposition to the Conservative Government and its plans for billions of pounds of cuts.
Comedian Russell Brand and singer Church both received loud cheers as they spoke at a rally at the end of the protest - the biggest in Britain for several years.
The Welsh singer, 29, described the idea that Britain needs austerity as "the big lie" and said: "What this country needs is economic stimulation - most economists around the world would say the same. We need to get the blood pumping."
She called for the country to rally and "save ourselves from decades of yuppie rule".
She attacked the "scandalous" sell-off of the bailed-out Royal Bank of Scotland and warned the Government will "sell off our schools and our hospitals, and once it is done, it will be very difficult to reverse".
She told the crowd: "One aspect of this that really gets under my skin is that it's all wrapped up in a proud-to-be-British package. I'm proud to be British because of our National Health Service, the welfare system and David Bowie. Not because of the Union Jack."
Earlier, she said: "I'm here today in a show of solidarity with everyone here - it is a massive turnout - everybody who thinks that austerity isn't the only way and thinks it is essentially unethical, unfair and unnecessary."
Organisers had promised a "festival atmosphere" and the march kicked off to the sounds of drum bands.
But a loud boo erupted through the crowd as it passed Downing Street and a red flare was set off, filling part of Whitehall with thick scarlet smoke.
Protesters, some clad in goggles and with scarves wrapped around their face to conceal their identity, chanted their opposition to Prime Minister David Cameron.
Another demonstrator waved a model of Margaret Thatcher's head on a stick.
But fears that violence would erupt at the protest, organised by the People's Assembly Against Austerity, appeared to be unfounded.
The Metropolitan Police said they had not made any arrests at the march, which was attended by many trade unionists and public sector workers.
Sian Bloor, 45, a primary school teacher from Trafford, near Manchester, said cuts are having a huge impact on children, with teachers resorting to bringing in food and clothes for their pupils.
She said: "Children come into school concerned because they are being thrown out of their house and have nowhere to live for the umpteenth time that year because their parents' benefits are being cut.
"They are being robbed of their childhood."
Organisers said an estimated 250,000 people were on the march in London, while demonstrations were also held in Glasgow and Liverpool.
Brand was received with loud cheers as he took to the stage to speak after left-wing Labour leadership hopeful Jeremy Corbyn.
The comedian spoke of his commitment to the anti-austerity movement and joked about his decision to back Labour days before the general election.
The 40-year-old, who had previously dismissed the idea of voting, said: "I think like most of us here I feel inspired to see people in this square in such incredible numbers after the results at the general election.
"Like most of you here I felt crushing disappointment on the morning after the election, unlike many of you I felt personally to blame for it. I thought I'd broken the country.
"My personal feelings about this movement are very, very deep.
"Without a welfare state I wouldn't have been educated, without a welfare state I wouldn't have had anywhere to live, without a welfare state my mum would've died of cancer several times.
"I am personally a product of the welfare state - not least because I signed on for eight years while I learned to be a comedian, so somewhat grateful for it."
At one point Brand asked the female sign language interpreter on stage with him if he was speaking too quick.
When she carried on using sign language, he said: "Thank you, what did that mean? That looked very explicitly like 'jiggle boobs'."
As he left the stage he was sent off with shouts of "Legend", with one female protester yelling: "Take your shirt off Russell."
Afterwards he posed for selfies with fans backstage but declined to be interviewed, saying: "I'm not working today."