The protesters blew horns and whistles as they marched past the Houses of Parliament in the capital on Saturday. Some carried banners reading political messages such as "austerity is failing" or "tax the rich, teach the poor."

"Austerity isn't working. It is hammering the poorest and the most vulnerable," Brendan Barber, the head of the umbrella group Trades Union Congress, (TUC) said.

London's Metropolitan police have not released an official estimate, but TUC organizers said Scotland Yard had told them that around 100,000 had attended the rally.

During a speech at the rally, the leader of the opposition Labour Party accused Prime Minister David Cameron of governing in the best interests of the wealthy, and at the expense of those who are not as well off.

"It is one rule for those at the top and one rule for everyone else," Ed Miliband said. "They cut taxes for millionaires and they raise taxes for ordinary families."

While this statement was greeted with cheers, there were some boos when Miliband added that even a Labour government would have been forced to make “hard choices.”

Deficit-cutting measures

Cameron's Conservatives, who govern in a coalition with their junior partners, the Liberal Democrats, argue that the spending cuts and tax hikes are necessary to wrestle down the country's deficit, which stood at eight percent of gross domestic product in 2011.

The prime minister responded to the protest through a message posted on the micro-blogging website Twitter.

"Today Ed Miliband is headlining a rally calling for an end to every single spending cut needed to clear the deficit," Cameron said.

Although Saturday's protest was largely peaceful, police said they had made a few arrests related to antisocial behavior in London's Oxford Street shopping district.

A similar rally in Glasgow is reported to have attracted around 5,000 people, while there was no word on how many turned out to a march in Belfast.

pfd/slk (Reuters, AFP, dpa)