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.
Among them were comedian Russell Brand and singer Church, who brandished an End Austerity Now placard.
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."
Asked if she was inspired by the surge of the Scottish National Party she said "absolutely".
The 29-year-old added: "But I think that the Scottish have been able to galvanise themselves against the Westminster elite. "We are in one of the richest nations in the world and social inequality is unacceptable. "I'm immensely proud to be here. I think this is a brilliant movement and it is for the common good. We are here to make a stand."
Speakers including Labour London Mayoral hopeful Diane Abbott addressed the crowds before they set off for the Palace of Westminster. Organisers 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 arrived outside 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. Many trade unionists and public sector workers were among the crowd. Sian Bloor, 45, a primary school teacher from Trafford, near Manchester, warned that children "are being robbed of their childhood" because of swingeing Government cuts.
She said: "We have seen a huge impact on our work at primary school. "I regularly bring clothes and shoes for children and biscuits for their breakfast, just so they get something to eat. "You can see how children are being affected by the cuts.
"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."