Doctors have joined picket lines at NHS hospitals across the country as they begin the first all-out strike in the organisation's history.
There were several dozen medics in place at King's College Hospital in Denmark Hill, south London, for the walk-out's scheduled 8am start.
They put up banners with slogans including "Junior doctors striking to save your NHS" and "5 into 7 doesn't go".
There were supportive beeps and shouts from several passing cars and cyclists, and one motorist who shouted "get back to work" as he went by.
Accident and emergency Dr Tom Roberts, 28, said he was striking because of the "unfair and unsafe" contract proposed by Health Secretary Jeremy Hunt that would leave many doctors "underpaid and over-stressed".
Dr Roberts, originally from Perthshire, told the Press Association: "We believe this contract will spread doctors too thinly across the ground and the NHS, from August if he goes through with this, will be unsafe for everybody.
"The BMA and junior doctors ... have asked Jeremy Hunt on so many occasions to just come and talk again, and he just tweeted responses to well-meaning questions.
"It is very, very frustrating for us. He just doesn't want to face doctors, he doesn't want to talk to us."
John Moore, a junior doctor in Newcastle, said: "I think Mr Hunt has mishandled this dispute from day one. He has refused to listen to any advice, even from his own MPs and researchers.
"He's misused statistics, he's outright lied, he's worried the public into not attending the NHS when they need to, and I think he should resign.
"He's completely lost the trust of an entire profession so unfortunately I don't see a route back to negotiations with him.
"But we are always happy to negotiate as we don't want to go on strike, so when there is a new health secretary I'm sure we will have more fruitful negotiations with them and I'm confident a deal can be reached.
"Let's not forget the BMA reached an agreement with the NHS employers but Jeremy Hunt personally vetoed that deal - he is the problem."
Haematology registrar Dr Natalie Heeney said that not only would the contract lead to compromises in safety, but it could also have a negative impact on research.
Many doctors in training take time out before becoming a consultant to do research. But because medics will not want to be on the new contract for a long period of time, they are less likely to do this, she said.
Speaking on the picket line outside Guy's Hospital in London, Dr Heeney also said that some medical specialities will suffer because junior doctors will opt to practise in areas with shorter training periods.
"I have got a couple of colleagues who are at the start of their haematology training and they have said that they will probably retrain for a different speciality that takes less time to complete the training," she said.
"For example, once you are on the general practice training scheme it takes three years to train whereas the majority of hospital specialities take a minimum of five years.
"And in a lot of specialities you take time out to do research, which will be an additional three years. So that could be eight years before you complete your training compared to three years to become a GP.
"I suspect that more people will apply for general practice because they won't have to work in a hospital as long and be forced to work this contract.
"I definitely think research will suffer."
King's College first-year doctor Benedict Porter, 26, accused the Health Secretary of having an "ulterior motive" because the elective services he wants to happen at weekends "are more lucrative but they don't save lives".
Dr Porter, from Stockwell, south London, said: "We believe that Jeremy Hunt is packaging up the NHS so it can be bought into by private companies by providing more lucrative services across seven days. We believe he has an ulterior motive.
"We are already overstretched, we already work long hours. We love what we do and we are happy to do that, but if you want the same number of doctors and to spread them across the seven days, the service is going to be at breaking point."
About 20 medics at the Maudsley Hospital psychiatric unit across the road from King's College Hospital are also on strike.
Psychiatric registrar Dr Jacob Bird said the new contracts will include "severe" cuts to payments to doctors who are called out to non-residential mental health cases.
He said: "Mental health is somewhat neglected already.
"My concern is that with the imposition, the forcing-through of this contract, less doctors will go into psychiatry because of the impact it will have, particularly on non-residential on-call (doctors)."