Labour leader Jeremy Corbyn urged tens of thousands of demonstrators to “defend the NHS with all of your might”.
He made his rallying call to cheering crowds, of all ages, at the national demonstration to oppose NHS cuts.
He told them in a speech in Parliament Square: “Defending the NHS is defending a basic human value and a basic human right.
“You don’t walk by on the other side when somebody is in difficulties or needing help.”
He called for next week’s Budget to properly fund the health service, social care and crucial mental health services.
He said: “The NHS is in crisis, in crisis because of the underfunding in social care and the people not getting the care and support they need.
“There are those waiting on trolleys and those who are desperate to get into an A&E department waiting hours for treatment.
“It is not the fault of the staff. It is the fault of a Government who have made a political choice.”
Organisers said the national demonstration is particularly needed now because “more austerity in the NHS represents a real risk to the safety of patients and the service”.
Backing came from presenter Sue Perkins who tweeted: “Love & support to all those marching for the NHS today, with a special shoutout to KONP, West Cornwall Health Watch & the gang in Penzance.”
Unions, frontline staff and children were among those who took part as the march peacefully moved from NHS England, through London’s West End and into Westminster.
Organisers said that 250,000 people were at the protest. Scotland Yard said there had not been any arrests.
Len McCluskey, general secretary of the Unite union, tweeted: “I’m marching because I am furious. Tories destroying the greatest gift the people of this country have created #SaveOurNHS.”
He said: “What an unbelievable demonstration we have been on here – 250,000 people from all over our nation fighting to save our national health service.”
Protesters came dressed for the occasion. A doctor held a banner which read “The blood on my hands washes off,” and a member of the public had mocked up a head wound with blood dripping down her face.
Messages on home-made banners read “SOS England’s NHS” while messages such as “NHS4Ever” were written on T-shirts , bags and umbrellas.
Mark Serwotka, the general secretary of the Public and Commercial Services Union, made his first public speech since having a heart transplant at Papworth Hospital in Cambridgeshire last year.
He told the crowds: “I wouldn’t be here today and neither would many, many others if it wasn’t for the dedication of the most marvellous public sector workers we have known, who are our NHS staff.”
After contracting a virus in 2010, his heart failed and over the last two years he has been kept alive by a mechanical pump.
Mr Corbyn, shadow chancellor John McDonnell and Mr McCluskey all welcomed him back to grassroots campaigning.