Video report by ITV News' Carl Dinnen
Up to 20,000 people have marched against new contracts for junior doctors, as the Government pushes ahead with its plans for a seven-day NHS.
Critics say the contracts would cut pay by up to 40% and force the trainee doctors to work longer hours - which many say put patient safety at risk.
Jeremy Hunt has denied the contracts would reduce pay going to junior doctors and insisted working hours needed to change to provide adequate weekend cover. He also accused the British Medical Association of "misrepresenting" government plans to doctors.
Protesters chanted "Hunt must go" and "Not safe, not fair, Jeremy Hunt doesn't care" as they made their way to Parliament Square.
Among the speakers at the rally was 92-year-old activist, Harry Leslie Smith, who moved some doctors to tears with his emotional address.
He told the crowd the NHS is "Britain's greatest achievement" that has freed millions from sickness. He warned that it must not be handed over to corporations.
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.