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 are set to join 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).
Doctor Sheneen Meghji, 32, a junior doctor from London, 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."
Ms Meghji, who works at Southend University Hospital, held a sign which said: "Save our NHS!" 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.
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 will move along Pall Mall and Whitehall.
It will end in Parliament Square outside the House of Commons between 5pm and 6pm.
Protests in Belfast and Nottingham will also take place on Saturday, and in Dundee on Sunday.
A huge crowd has gathered in Waterloo Place to listen to a number of speakers. Their cheers can be heard in Trafalgar Square and hundreds of them are holding banners and placards, with many wearing medical clothing.
Signs carry messages such as "Save Our NHS" and "Bad decisions cost lives". Others say "Tired doctors make mistakes" and "Quantitative easing for doctors not bankers". One sign held high said: "Cunning Hunt's conning health care. "