Dr Rob Galloway addressed a crowd of up to 20,000 people, including a large group from Brighton, at the demonstration in London.
The accident and emergency consultant at the Royal Sussex County Hospital encouraged people to “fight together” against the plans, warning that the “NHS is in peril”. Critics say the plans announced by health secretary Jeremy Hunt will heap pressure on doctors, risking the quality of the service.
Demonstrators at the rally on Saturday, which went past government buildings from Waterloo Place to Parliament Square, chanted “Hunt must go” and “Not safe, not fair, Jeremy doesn’t care” .
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 per cent 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.
After the rally, Dr Galloway said: “The NHS is on a knife-edge and could get much worse by an unfair and unjust junior doctor contract.
“We need to force the government to rethink the way they are treating the back bone of the NHS – junior doctors.
“Otherwise they may leave and we will not be able to care for our patients.”
Medical student Amaran Cumarasamy was among the doctors, students, staff and supporters from Brighton who took part in the rally.
The 24-year-old said: “The British public are tremendously proud of their NHS, but cannot expect world-class care without the right level of infrastructure there.”
Mr Hunt accused the British Medical Association, the doctors’ trade union, of ”misrepresenting” the proposed changes, insisting the new contract is a ”good deal” for doctors.
He told the BBC: ”It’s caused a huge amount of anger unnecessarily. We don’t want to cut the pay going to junior doctors, we do want to change the pay structures that force hospitals to roster three times less medical cover at weekends as they do in weeks.”