Jeremy Corbyn joined thousands of junior doctors as they marched through Whitehall following a day of strikes.
Leading the march from St Thomas' Hospital and flanked by shadow chancellor John McDonnell, he said: "The Government has an opportunity to settle this, they should get on and do so."
McDonnell said Health Secretary Jeremy Hunt would eventually have to back down amid sustained pressure from junior doctors.
Supported by teachers' unions and the BMA, thousands of junior doctors marched to the Department of Health to make their anger known, chanting: "Where are you Jeremy?"
Corbyn was roundly cheered as he took to a stage to address the protesters outside the Department of Health.
He told them the NHS was not safe in the hands of a Government more interested in attacking the junior doctors at its core than supporting it, selling off its assets and destroying its "very principles".
Corbyn said it was "utterly contemptible" that Hunt had failed to reach an agreement with junior doctors, despite having "every conceivable opportunity".
He said: "They are the ones who have behaved in the responsible manner of saying they are there to defend the NHS. His response is to try and impose a contract and impose something on them.
"That is no way for a secretary of state to behave towards one of the most crucial elements of the NHS workforce."
Amid a chorus of boos - that quickly turned to cheers and laughter - he added: "Don't worry, not all Jeremys are bad."
Doctors wearing medical scrubs were joined by scores of supporters as they thronged across Westminster Bridge in central London on their march to Whitehall.
The National Health Singers, the NHS's choir, sang their song Yours as they walked, but the words were often drowned out by calls to "save our NHS" and vocal attacks on Jeremy Hunt.
Many carried banners and placards to make their anger known - one said "Jeremy you muppet, we already work weekends and nights", while another said simply: "More talks, less spin".
As they reached the Department of Health building on Whitehall, McDonnell said the strike could have been settled on Monday through negotiation but that Hunt had refused to meet the junior doctors.
He said: "There is a solution here - he needs to sit around the table with the doctors and negotiate sensibly."
Responding to the suggestion that Hunt will not back down, McDonnell said: "Eventually he is going to have to, because the junior doctors are adamant that this contract will put patients at risk and they are not willing to stand by and let that happen.
"He has got to start negotiating again. The whole community is now calling upon him to get back around the table."
Addressing the protesters from the stage, he called junior doctors the "heroes and heroines of the struggle to save the NHS".
Thanking them for their efforts to keep patients safe, he added: "I want to thank you for having the courage to stand up to Hunt ... you have shown the way.
"Let's make it absolutely clear to Jeremy Hunt, we are not going away until we have reached a just and negotiated settlement that protects people's jobs, their livelihoods, the hours they work, their pay and their conditions and above all else protects the patients.
"We are going to win this battle, and we are going to win it through courage and determination."
And speaking to the teachers who joined the protest to support the junior doctors, he said: "If and when they come for you, we will be with you as well.
"We will not allow this government to dismantle our welfare state brick by brick, service by service."