Slogans such as “Keep your dirty hands off our hospitals” and “cut to the bone” were held aloft by the chanting crowds as they moved through the streets on Saturday afternoon.
The march, organised in protest against Government cuts to the NHS, privatisation and the closure of hospitals, began in Tavistock Square at midday and weaved its way through the streets for a rally outside Parliament.
Labour leader Jeremy Corbyn, Bernie Sanders’s brother Larry, Unite general secretary Len McCluskey and Coronation Street stat Julie Hesmondhalgh were among those due to give speeches at the rally in Parliament Square.
Aerial photos revealed the scale of the crowds who joined the march, which had been billed as the UK’s biggest ever NHS demonstration.
Campaigners were in high spirits as they used megaphones and whistles to make themselves heard as they made their way through the capital.
In a message to the health secretary, they could be heard chanting: "Jeremy Hunt, we are here to say, our NHS is here to stay",
Messages such as "NHS4Ever" were written on t-shirts, bags and umbrellas.
Mr Corbyn urged the campaigners to "defend the NHS with all of your might".
He made his rallying call to cheering crowds, of all ages.
He told them in his speech: "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."
Demonstrator Felix Ramos, 53, of Hackney, east London, stood next to a coffin on the march.
He said: "Many people are not going to get the help they need. If you do not have health, there is no life.
"Privatisation does not work for life. It is not going to care for the vulnerable but it might care for people who can afford special treatment. It is not for the majority."
Dr David Wrigley, a GP from Carnforth, Lancashire, and deputy chair of the BMA council, was among the front-line staff marching under the slogan "Our NHS, No Cuts, No Closures, No Privatisation".
He said: "Today's march is a cry for help for anyone who uses the NHS because it is in such a desperate situation. We need to highlight it.
"As a doctor I see day-to-day the serious pressures in the NHS due to the funding cuts from the Government.
"Patients are not getting the care they deserve. We are a country that can afford the funding that is required."
The campaigners, who say the NHS is at breaking point, hope the march will put pressure on the Government ahead of next week's Budget.
Dr Wrigley added: "I guess it is also a wake-up call for Theresa May and the Chancellor... we demand they fund the NHS adequately. If the Budget does not bring about any further funding increase, there would be uproar."
Junior doctor and GP trainee Dr Jeeves Wijesuriya believes the demonstration is a chance for the Government to plot a new course for the NHS.
Speaking on behalf of the BMA, he said the NHS "in reality faces £26 billion of health and social care cuts due to political decisions".
He told the crowd: "What is sustainable about debts that can only be paid with our patients' health and yet still won't be settled?
"Where is the transformation when the money to build new hospitals and health centres is being siphoned off to pay debts?"
Many of the people taking part shared their own photos on social media.
NHS employees including doctors, nurses, midwives and radiographers travelled from all over the country to attend the #OurNHS march organised by The People’s Assembly and Unite the Union.