Organisers claimed 250,000 people joined a protest against the government's cuts to the NHS on Saturday (4 March).
Coaches were put on across the the UK and will deliver people to London ahead of the march, which is to start at 12pm GMT.
The protest will look to highlight the increasing pressures on staff faced with dwindling resources and an ever-ageing population, as demonstrators march from Tavistock Square to the Houses of Parliament.
A spokesman for Unite the Union, one of many unions and campaign groups taking part, said the march aimed to send a strong message to Prime Minister Theresa May and Chancellor Philip Hammond ahead of this week's budget announcement on 8 March.
"We must fight to save our NHS from destruction," the spokesman said. "The threat is real. It is happening now.
"Hospitals, GPs, mental health, ambulance and community services are on their knees.
"Private companies are gaining an ever greater foothold within the NHS. Years of pay restraint has seen the value of NHS staff salaries reduce by 14% since 2010."
The spokesman added: "The NHS is one of our greatest achievements. We cannot allow it to be undermined and ultimately destroyed.
"Our NHS needs you to fight for it. We must march together, sending a clear message to the government, 'the NHS will last as long as there are folk with the faith to fight for it'."
Unite's national officer for health Sarah Carpenter told the Mirror: "The NHS, which is underpinned by a brilliant and dedicated workforce, has struggled throughout this winter.
"The evidence that the health service is starved of cash has been horrific with patients waiting on trolleys for treatment.
"When Philip Hammond gets up to deliver his budget next week, he needs to open his chequebook and be generous to the NHS – it is what the British public wants."
Labour leader Jeremy Corbyn is due to speak at the demonstration, in addition to the Green Party's health spokesperson, Bernie Sanders' elder brother Larry Sanders and Coronation Street star Julie Hesmondhalgh.
Corbyn will call on the government to guarantee the rights of three million EU nationals living in Britain, stating that they are an essential facet of Britain's public health service.
Organisers said over 250,000 people were in attendance at the march. London Metropolitan Police told IBTimes UK that it could not confirm or deny the figure, stating that the force does not comment on the size of protests.