Jeremy Corbyn has released a video in support of the thousands of people marching in London demanding “health, homes, jobs and education”.
He said in the post on his Facebook page: "Tens of thousands of people have taken to the streets in London today for the People's Assembly March for Health, Homes, Jobs and Education.
"I couldn't join them as I'm campaigning in Liverpool for Labour victories in the local elections on Thursday 5 May.
"But, I sent this video to the demonstration and I'm taking the same message around the country - austerity is a political choice, not an economic necessity.
"The Labour Party is now an anti-austerity Party and we're standing up against the failed policies of the Tory government."
Slogans such as Cameron Must Go - Tories Out! and demands for decent health, homes, jobs and education are being brandished in the protest organised by the People's Assembly.
The march began by the University of Central London and is currently weaving its way through the streets for a rally in Trafalgar Square, reports the Press Association.
Kicking off the rally, the National Health Singers sang a song they had written, which included lines of "don't let our junior docs be worked around the clock", and "help us keep you safe, don't take our rights away".
Before the crowds set off, Labour shadow secretary for international development, Diane Abbott, spoke to the gathering.
The Hackney North and Stoke Newington MP said: "Fighting austerity is the political struggle of our time.
"It is austerity that is the real threat to the NHS. It is austerity which is stopping local authorities building homes.
"It is austerity that is forcing people out of work and into zero hours contracts.
"It is austerity that threatens the future of our young people.
"There could not be a more important demonstration or a more important movement than this one."
She told the crowd that this is "probably the biggest demonstration ever," and is "the right cause".
Chris Nineham, of Stop The War Coalition, said: "Austerity is not about economic necessity, it is a political choice."
He added: "We can now not just get rid of David Cameron, but the whole rotten Tory Government."
Labour's shadow chancellor John McDonnell, Unite general secretary Len McCluskey, NUT general secretary Christine Blower and Green Party leader Natalie Bennett are also joining the demonstration.
They will be speaking at a rally in Trafalgar Square at the end of the march.
More than 100 coaches filled with demonstrators have arrived in the capital from around the UK - with thousands of others attending through their associated unions or groups.
It is estimated that more than 50,000 are taking part in the demonstration.
Michaella Hagger has travelled to the demonstration from Winchester.
The 27-year-old who works in the probation service, said: "I'm here because I hate David Cameron.
"It's all about the cuts, tax dodging, and the NHS for me. They are ruining people's jobs and making it impossible for everyone."