Union activists have taken part in a series of protests across the UK as part of a campaign against the government's spending cuts.
Rail, Maritime and Transport (RMT) union general secretary Bob Crow told a London rally collective action was needed to fight the cuts.
It comes after the TUC said a national demonstration will be held on 26 March next year in London's Hyde Park.
Chancellor George Osborne announced deep public spending cuts on Wednesday.
About 490,000 public sector jobs are likely to be lost as part of the cuts - the deepest and most wide-ranging by a government in decades.
The government says the cuts are necessary to reduce the UK's £155bn deficit and strengthen the UK's economy in the long term. It aims to save £83bn in four years.
During his announcement, Mr Osborne said: "To back down now and abandon our plans would be the road to economic ruin," adding that "a stronger Britain starts here".
Most of the demonstrations throughout the UK passed peacefully, but two people were arrested and two police officers were injured at a march organised by the recently-established Bristol and District Anti-Cuts Alliance in Bristol.
In London, demonstrators gathered outside the RMT head office to hear speeches from Mr Crow and Matt Wrack, leader of the Fire Brigades Union, which is also holding a strike in London.
'Wave of protest'
Mr Crow told demonstrators the movement needed to be built up "into the housing estates, into the workplaces, into every part of society and say we aren't paying the price for the corruption of the bankers".
He called for collective action, saying: "When London Underground workers take strike action it shouldn't just be about Underground workers, it should be about the three million people that use it.
"When firefighters take action, it shouldn't just be about the firefighters, it should be about the people who rely on their services.
"And when public sector workers and private sector workers fight over the next 12 months to defend their jobs and their communities, we've got to build up the biggest resistance possible."
He predicted the UK-wide rallies would "kick-start a tidal wave of protest".
In Edinburgh, 20,000 people from all across Scotland marched through Princes Street before a rally at the Ross Bandstand in Princes Street Gardens.
At the front of the march, Scottish Labour leader Iain Gray said: "The turnout at this march shows that the people of Scotland do believe there is a better way. The coalition cuts are too deep and too fast, putting 100,000 jobs at risk."
Other prominent politicians at the rally - which was arranged by union group the STUC - included Justice Secretary Kenny MacAskill.
Before the march, STUC general secretary Grahame Smith said he wanted "to dispel the myth there is no economic alternative to these cuts".
"There is an alternative. Get people back to work, get the economy growing again, and the public finances will largely take care of themselves," he said.
George Samson, of the Dundee Pensioners Forum, said he had travelled to the event to defend public services.
"Pensioners depend on these services and these cuts that are forthcoming are going to affect us adversely," he said.
Union members also protested in Cardiff and Wrexham.
Meanwhile, Social Development Minister Alex Attwood and Education Minister Catriona Ruane were among the several thousand people that attended a rally in Belfast.
Workers, students and trade unionists also gathered in Norwich for an event staged by the Norfolk Coalition Against the Cuts, while the Yorkshire and Humber TUC held a rally in Sheffield.
In Cambridge, demonstrators marched through the city before attending a rally in the Guildhall.
The decision to hold a national demonstration in March 2011 came after moves within the TUC to organise this protest before the end of the year were overruled by other, less hardline, union leaders.
Earlier this week health workers, council staff, firefighters, teachers and other public sector employees held protests ahead of the cuts announcement.
Thousands in Westminster cheered speeches by union leaders before lining up to lobby MPs about cutbacks.