Anti-austerity marches were held in London, Glasgow and Belfast on Saturday.
The Trades Union Congress estimated that more than 150,000 people took part.
The government says austerity measures are vital to cutting the deficit.
Opposition leader Ed Miliband was booed in Hyde Park at the end of the march when he suggested there would still be spending cuts under Labour.
The BBC reports the marchers were brought to London in more than 250 coaches. They assembled on the north bank of the Thames and set off at about noon.
Others came from Wales in a chartered train.
Marchers in Glasgow came from all over Scotland and the north of England.