Tens of thousands of people have marched in protest at the government's austerity measures.
Labour leader Ed Miliband, among dozens of speakers who addressed crowds at the biggest march in London, received a mixed reaction. Other rallies took place in Glasgow and Belfast.
The government says austerity measures are vital to cutting the deficit.
Away from the rally, the Met Police said it was responding to anti-social behaviour in the Oxford Street area.'Wages falling'
Organiser Trades Union Congress (TUC), which said workers and campaigners from across the UK were involved with the demonstration, estimated that 100,000 people were 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 - Union leaders recently criticised Labour for supporting a public sector pay freeze.
"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 - brought to London in more than 250 coaches - had assembled along Victoria Embankment on the north bank of the Thames from 1100 BST and set off at about noon.
Banners on display included those reading "Cameron has butchered Britain", "no cuts" and "plebs on parade".
In a separate development, the Met Police said it had responded to a number of incidents of anti-social behaviour in the Oxford Street shopping area and that some arrests had been made.
At the March 2011 demonstration, there were clashes between police and protesters in Trafalgar Square - hours after a peaceful march to Hyde Park. A total of 201 arrests were made that day.