The march is the second of its kind in the capital as the row continues between Health Secretary Jeremy Hunt and representatives of the British Medical Association (BMA).
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 will start with speeches at 2pm at Waterloo Place, before the march moves 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 today, and in Dundee on Sunday.
Dr Anna Warrington, of the protest organising committee, said: “This unprecedented protest brings together healthcare professionals and concerned public to raise awareness of the threat to our NHS from the imposed junior doctors’ contract.
“Junior doctors will not agree to contract changes that risk patients’ safety and doctors’ wellbeing. This event is about explaining our concerns to a wider audience.”