The march began with a festive mood. Red flares exploded as the rally got underway at the Bank of England, sending plumes of scarlet smoke through the financial district's narrow and winding streets. Brass musicians offered a tune as demonstrators waved placards, balloons and banners on the way to Parliament Square.
The event is intended to vent outrage, albeit peacefully, over the Conservative government's plan to slash millions of pounds in public spending to address the deficit, which ballooned after the nation rescued its troubled banks during the 2008 financial crisis.
Social activists, union leaders and campaign groups joined forces to protest cuts in public services, welfare and education that are expected when the government unveils its budget next month. Demonstrators argue the public is being punished for a crisis it did not cause.
"I am fed up with all the main parties saying they are going to promote austerity," Lindsey German, a senior official of the Stop The War Coalition. "They have made the rich richer and the poor poorer and they're going to continue."
Singer Charlotte Church brandished an "End Austerity Now" placard and said she wanted to take part in solidarity with others who believed cuts had gone too far. Comedian Russell Brand was also among the thousands taking part.
"It will be the start of a campaign of protest, strikes, direct action and civil disobedience up and down the country," said Sam Fairbairn of the People's Assembly, which is organizing the protests. "We will not rest until austerity is history, our services are back in public hands and the needs of the majority are put first."