Tens of thousands of people are marching in protest at the government's austerity measures.Labour leader Ed Miliband, among dozens of speakers addressing crowds at the biggest march in London, received a mixed reaction. Other rallies are taking place in Glasgow and Belfast.
Demonstrators want the coalition to end public service cuts and instead create policies they say can create growth.
The government says austerity measures are vital to cutting the deficit.
Union leaders recently criticised Labour for supporting a public sector pay freeze.
'Wages falling'Organiser Trades Union Congress (TUC), which says workers and campaigners from across the UK are involved with the demonstration, estimates that 100,000 people are taking part while the Met Police has not released an estimate.
That compares with the more than 250,000 people who took part in a London anti-cuts march and rally in March 2011.
Mr Miliband, speaking at a rally in Hyde Park at the end of the march, said his party was there for "all the young people in this country who want work, but can't find it in Britain today".
He was booed when he suggested there would still be spending cuts under Labour.
"I have said whoever was in government now there would be some cuts, but this government has shown that cutting too far and too fast, self-defeating austerity is not the answer, it is not the answer to Britain's problems," he said.
But there were cheers when he referenced the incident in which Chancellor George Osborne had to pay for an upgrade after he sat in a first class train carriage with a standard class ticket.
He was also cheered when he spoke about Andrew Mitchell's resignation after the chief whip admitted swearing at police officers during a Downing Street confrontation, but denied calling them "plebs".
"Andrew Mitchell may have resigned, but the culture of two nations runs right across this government," Mr Miliband said.
"They cut taxes for millionaires and they raise taxes for ordinary families."
The marchers had assembled along Victoria Embankment on the north bank of the Thames from 1100 BST and set off at about noon.
Banners on display have included those reading "Cameron has butchered Britain", "no cuts" and "plebs on parade" - a reference to the row that led to the resignation of government chief whip Andrew Mitchell.
Demonstrators were brought to London in more than 250 coaches.
Elsewhere around the UK:
- The Wales TUC chartered a train and coaches so that people from all over Wales could march in London.
- The BBC's Laura Maxwell, at the Glasgow march, said the people there had come from all over Scotland and the north of England to add their voice to the national demonstration. Most of the criticism there is being levied at the Westminster government, but the Scottish TUC says the Scottish government has to stop hiding behind Chancellor George Osborne's public sector pay cuts, our reporter added.
- At the Belfast rally, organised by the Northern Ireland Committee of the Irish Congress of Trade Unions (Ictu), assistant general secretary Peter Bunting said Stormont must not "slavishly" cut back on the say-so of the government in London. He said workers must not allow themselves to be divided into sectarian blocs, as they had been in the past.
However, TUC general secretary Brendan Barber said: "The evidence is mounting that austerity is failing.
"More than 2.5 million people are out of work, a further three million are not working enough hours to make ends meet, and wages have been falling every month for the last three years."
He told BBC Radio 4's Today programme the "huge squeeze on wages and living standards" had led to a "massive hit on confidence and on demand in the economy".
"That's why some of our biggest companies that are sitting on big cash reserves aren't investing that and getting our economy moving again."
However, he said he did not think a general strike by unions was likely, adding: "Some of my colleagues may talk about that. I don't hear too many people calling for a general strike."
Calls for a mass walk-out over spending cuts have grown in recent months, with the TUC Congress voting in September to look into the practicalities of organising one.
A government spokesman said: "It is disappointing that some unions insist on pushing for irresponsible and futile strike action which benefits no-one. As we have said time and again, pension talks will not be reopened and nothing further will be achieved through strike action."
Conservative Party Chairman Grant Shapps said of Ed Miliband: "You can't be serious about clearing the deficit when you attend a march that calls for an end to austerity."