In possibly the biggest protests since those against the Iraq war in February 2003, organizers say up to 250,000 people took to the streets of London on Saturday to show their frustration with planned austerity measures designed to cut a record budget deficit.
Police would not confirm that number. People carried banners reading Don't break Britain and No to cuts in the largely peaceful protests, with about 4,500 police standing by.
The protests were characterized by an almost carnival atmosphere, with dancers, choirs, bagpipers and the sound of vuvuzelas providing the backdrop.
Some anarchist demonstrators set off flares and fireworks, however, and a small breakaway group threw bottles and paint at a branch of the Royal Bank of Scotland, one of the UK's largest banking groups.
Masked demonstrators smashed the windows of clothing chain Topshop on Oxford Street in central London and there were sporadic scuffles with police.
The March for the Alternative, was organized by Britain's umbrella union body, the Trades Union Congress (TUC), to protest plans by Britain's Conservative-Liberal government to slash public services to the tune of £ 81 billion pounds (130bn USD).
The cuts are an attempt to almost eliminate by 2015 the country's soaring budget deficit, currently 10% of GDP, which the current government blames on the previous Labour administration.
Unions and the opposition Labour Party say the measures go too far, too fast and bring misery to millions in Britain, where unemployment is at its highest level since 1994.
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 Labour leader Ed Miliband told demonstrators in London's Hyde Park.
The government, meanwhile, insists the cuts are painful but necessary.
Of course, people will feel a sense of disquiet, in some cases anger, at what they see happening, Education Minister Michael Gove told BBC Radio.
But the difficulty we have, as the government inheriting a terrible economic mess, is that we have to take steps to bring public finances into balance he said in a thinly veiled swipe at the previous Labour government.
The demonstrations follow student protests last year against a tripling of university tuition fees. Several of those demonstrations turned violent, with one resulting in protesters damaging the car carrying heir-to-the-throne Prince Charles and his wife Camilla.