Tens of thousands of people are marching through the streets of London
Tens of thousands of people are marching through London in protest against the government's austerity measures.
Labour leader Ed Miliband is among dozens of speakers due to address crowds in Hyde Park - where the march ends, reports the BBC.
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.
The Trades Union Congress (TUC), said workers and campaigners from across the UK would be involved with the demonstration. Other rallies are taking place in Glasgow and Belfast.
The London march assembled along Victoria Embankment on the north bank of the Thames from 1100 BST and set off at about noon.
Demonstrators were brought to London in more than 250 coaches.
Earlier this month, 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 2.5m 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.
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.