Thousands of people are marching in protest at the government’s austerity measures.
Labour leader Ed Miliband is among dozens of speakers due to address crowds at the biggest march in London. 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’The Trades Union Congress (TUC), said workers and campaigners from across the UK would be involved with the demonstration.
Brendan Barber describes the government’s austerity measures as a “negative strategy”
The London march assembled along Victoria Embankment on the north bank of the Thames from 1100 BST and set off at about noon. The march ends in Hyde Park.
An hour and a half after it began, the Met Police tweeted that, while the head of the march was in Hyde Park, the rear of the march was only just moving along Victoria Embankment.
Demonstrators were brought to London in more than 250 coaches.
Elsewhere around the UK:
Earlier this month, Mr Cameron warned more “painful decisions” would be necessary to repair the UK economy, adding that he would not waver from austerity measures.
And on Saturday, around the time of the start of the London march, he posted a message on Twitter stating: “Today Ed Miliband is headlining a rally calling for an end to every single spending cut needed to clear the deficit #labourisntlearning.”
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.
‘Irresponsible and futile’Organisers are hoping Saturday’s march will repeat a mass demonstration in 2011 over controversial pension reforms, which was attended by more than 250,000 people.
Mark Serwotka, leader of the Public and Commercial Services (PCS) union, is expected to say: “Almost everyone now agrees that austerity isn’t working and that this government’s policies are making our economic situation worse, not better.”
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.”