Thousands of people – including NHS workers, campaigners and union representatives – are marching in London to protest against “yet more austerity” in the health service.
Protesters on the #OurNHS march want to draw attention to plans which could see services in nearly two-thirds of England cut back.
Union leaders say many NHS services “are on their knees”.
Labour leader Jeremy Corbyn is expected to address people in Parliament Square.
The march began outside private healthcare company Virgin Care in Tavistock Square, and will end with a series of speeches in Westminster.
The group claims that the government’s proposed Sustainability Transformation Plans (STPs) across the local NHS in England are a “smokescreen for further cuts and are [the] latest instruments of privatisation”.
Last month the BBC revealed that hospital services in nearly two-thirds of England could be cut or scaled back.
The proposals would see the complete closure of some hospitals and the centralising of some services such as AE and stroke care on fewer sites.
On Twitter Dr Ben White said: “#OurNHS sees you from the cradle to grave, with or without insurance. Sadly we must now fight to keep it that way.”
Labour shadow chancellor John McDonnell is one of a number of figures leading the march, which finishes at Parliament Square.
Unite leader Len McCluskey tweeted from the march, saying: “I’m marching because I am furious. Tories destroying the greatest gift the people of this country have created #SaveOurNHS.”
The union says “hospitals, GPs, mental health, ambulance and community services are on their knees”.
A Department of Health spokesperson said: “We are committed to the NHS which is why we’re investing £10bn in its own plan for the future, including £4bn extra this year to transform services and improve standards of care.”
Organisers of #OurNHS claim the march is the biggest protest against government healthcare policy in history.
From the scene: Richard Lister, BBC News
They arrived by the bus load this morning; 150 coaches brought protesters from across England and Wales.
They set off from Tavistock Square to the beat of drums, and serenaded by the Lewisham and Greenwich NHS Choir.
Some blew whistles, some chanted, one man was dressed as a “therapy dog” for overworked NHS staff.
Most are carrying “#OurNHS” placards; others said “Dump Trump”, “No to racism” and “Migrants make our NHS”.
One of the organisers, John Rees told me Brexit had consumed much of the “oxygen of debate” in recent months but the NHS was rising up the national agenda again.
London-based nurses Sebastian Birch and Danny Goddard told me problems had mounted under Labour and Conservative governments, to the point where they now see the crisis as a “social welfare issue” not just one of healthcare funding.