Thousands of people set off on coaches from Somerset, Gloucestershire, Wiltshire and Bristol early this morning.
They are heading to the Capital for a march and rally against the Government's austerity policies.
Demonstrators say they want an alternative economic strategy which puts "jobs, growth and people first" and stops public service cuts.
The government says austerity measures are vital to cutting the deficit.
'A Future That Counts' is organised by the TUC, the body that represents trade unions.
The focus of the march will be to secure a better future for the UK's young people. Organisers say one million young people are currently out of work.
Marchers will assemble on the Victoria Embankment in London from 11.00. The rally in Hyde Park starts at 13.00 and runs until 1600.
Labour leader Ed Miliband is among dozens of speakers due to address crowds in Hyde Park.