Health Secretary Jeremy Hunt has been accused of "peddling some vicious lies" at a protest in London over the Government's plans to impose a new contract for doctors.
Thousands of doctors joined a march - the second of its kind in the capital - as the row continues between Mr Hunt and representatives of the British Medical Association (BMA).
Up on 20,000 people were said to be taking part in the protest where demonstrators chanted "Hunt must go" and "Not safe, not fair, Jeremy doesn't care".
A series of letters between Mr Hunt and Dr Johann Malawana, head of the junior doctors' committee at the BMA, have failed to bring the two groups back to the negotiating table.
Under the current plans, the contract will reclassify doctors' normal working week to include Saturdays and late evening working.
Critics have argued the deal could mean pay cuts of up to 30%, with "normal hours" reclassified as being from 7am to 10pm, Monday to Saturday.
Extra payments for unsociable working will be earned only outside of these times, rather than the current arrangements of 7am to 7pm Monday to Friday.
Mr Hunt has indicated he would be willing to look again at how far "normal hours" extend on Saturdays.
The protest started at Waterloo Place, and a march moved along Pall Mall and Whitehall before finishing in Parliament Square.
A huge crowd gathered to listen to a number of speakers and their cheers could be heard in Trafalgar Square.
Banners carried messages such as "Save Our NHS" and "Bad decisions cost lives". Others said "Tired doctors make mistakes" and "Quantitative easing for doctors not bankers". One sign held high said: "Cunning Hunt's conning health care. "
Doctor Sheneen Meghji, 32, a junior doctor from London who works at Southend University Hospital, said the new contract plans are "really rather terrible" and are "dire" for the NHS.
She told the Press Association: "Jeremy Hunt is peddling some vicious lies to the media, to patients and to the public in general."
Peter Stefanovic, a partner in the clinical negligence department at Simpson Millar solicitors, gave a rousing speech which ended in the crowd chanting: "Hunt must go."
Asked about the new contract, he said: "It's an absolute public disgrace. An absolute public disgrace. The injustice staggers the imagination."
Ranj Singh, a 36-year-old junior doctor who works in paediatrics, also addressed the crowd.
Asked about the new plans, he said: "They're not fair and they're not safe. For anybody involved - that's patients and staff."
He added: "We want what is best for our patients."
One 92-year-old activist, Harry Leslie Smith, had doctors in tears at the end of his speech as they were overcome with emotion.
He told the crowd the NHS is "Britain's greatest achievement" because it has freed millions from sickness. He warned that it must not be handed over to corporations.
Alex Armitage, a 34-year-old paediatric registrar at Woolwich Hospital, who was at the front of the march, said of the plans: "It's an extension of the Conservative plan to set the NHS up to fail."
He said the plans make the NHS an "uninhabitable place for staff", adding that they are "just being squeezed and squeezed and squeezed".
Roshana Mehdian, a 29-year-old junior doctor, addressed the crowd through a loudspeaker as they sat down on Whitehall.
She said there were 20,000 people on the march and started a chant of "Where are you Jeremy?".
She told the Press Association: "We are protesting against an unfair and unsafe contract and this is showing our strength of feeling. We are here to show Jeremy Hunt that we will not accept that contract."
Strike action among doctors is still on the cards after negotiations broke down between NHS Employers and the BMA over the contract, which covers all doctors up to consultant level.
Protests in Belfast and Nottingham were also set to take place on Saturday, and in Dundee on Sunday.