Thousands of junior doctors across England are on strike in a row with the Government over a new contract.
More than 5,000 operations and procedures across England have been cancelled ahead of the 48-hour strike which began at 8am.
Health Secretary Jeremy Hunt has announced he will impose the contract on junior doctors - everyone up to consultant level - after months of talks with the British Medical Association (BMA) failed to reach a resolution.
Junior doctors will provide emergency care only on Wednesday and Thursday, with two further 48-hour strikes planned from 8am on April 8 and April 26.
The third strike by the BMA comes as an Ipsos MORI poll for BBC News found 65% of members of the public support the doctors' cause.
The survey found most people still think the Government is most at fault for the dispute, but a rising number believe equal blame should be shared by the Government and doctors' leaders.
Public support for the latest strike is as high as it was for the first two stoppages earlier this year, with 65% of 860 adults in England supporting the strike.
Some 57% said the Government was most at fault for the dispute continuing this long, down from 64% in February, while the number saying junior doctors were most at fault remained around 11%.
New figures from NHS England from 228 organisations, of which 154 are acute hospital trusts, show that 2,077 inpatient procedures have been cancelled due to Wednesday and Thursday's industrial action alongside 3,187 day case operations and procedures.
Hundreds more routine clinics and appointments are likely to be affected.
Dr Anne Rainsberry, national incident director for NHS England, said: "This is clearly going to be a difficult couple of days. A 48-hour strike will put significantly more pressure on the NHS and the cumulative effect of these recurring strikes is likely to take a toll.
"The safety and care of patients is always our number one priority and staff across the NHS are doing all they can to minimise the impact on patients of the action."
Urgent and emergency care services will be available as normal but hospitals are expected to be under extra pressure
Where possible, patients are being asked to contact their GP, seek advice from their local pharmacist, call 111 or check the NHS Choices website.
In an emergency, people should still call 999 or go to A&E.
The BMA is seeking a judicial review over imposition of the contract, though Government lawyers have argued this is "misconceived".
The major sticking point has been over weekend pay and whether Saturdays should attract extra "unsocial" payments.
Currently, 7pm to 7am Monday to Friday and the whole of Saturday and Sunday attracts a premium rate of pay for junior doctors.
The Government wanted the Saturday day shift to be paid at a normal rate in return for a hike in basic pay.
The BMA rejected this and urged Mr Hunt to reduce the offer of basic pay and instead have better premium rates on Saturdays.
The imposed contract, which is due to come into force in August, has an increase in basic salary of 13.5%.
Under the new arrangements, Mr Hunt said no doctor working contracted hours would see a pay cut while too many night shifts and long shifts will be limited.
But 7am to 5pm on Saturdays will be regarded as a normal working day.
A Department of Health spokesman said: "Patients have so far seen more than 19,000 operations cancelled as a result of the BMA's irresponsible and unjustified industrial action.
"The new contract, 90% of which was agreed with the BMA and endorsed by senior NHS leaders, is a very good deal for doctors and the NHS."
The Royal College of Radiologists said it remains "deeply concerned" about the impact on patients of the continuing dispute.
Johann Malawana, the BMA's junior doctor chairman, said: "We deeply regret disruption to patients, and have given trusts as much notice as possible to plan ahead, but the Government has left junior doctors with no choice.
"Ministers have made it clear they intend to impose a contract that is unfair on junior doctors and could undermine the delivery of patient care in the long term."
The Green Party offered its support to striking doctors.
Its health spokesman, Larry Sanders, who is the brother of US presidential hopeful Bernie, said: "The hypocrisy of Jeremy Hunt announcing that he was imposing a contract on junior doctors exactly one year after he 'called time on NHS bullying' is astounding.
"Those junior doctors are essential to a functioning NHS and they will go on to become leaders of the service. They need to be treated with the respect that they deserve.
"It is clear to all that Jeremy Hunt's agenda is to destroy the NHS and to pass it into private hands supported by an insurance scheme. He has gone on record with these views and he is therefore not a fit person to be in charge of the NHS."
Outside the Royal Sussex County Hospital in Brighton, passing motorists beeped their horns in support of the junior doctors on strike.
Orthopaedic registrar Christopher Gee said morale across the NHS was now at "an all-time low" amid uncertainty over the future of the NHS.
He said: "It is very frustrating. Doctors feel they have been backed into a corner.
"We all very conclusively believe we have no other choice but to strike. We are trying to do everything we can to get the Government to listen.
"The junior doctors' contract is a significant patient safety issue. There is a recruitment crisis in the NHS - doctors and nurses are already stretched.
"Trying to stretch services further when there hasn't been any proper planning is essentially dangerous for patients."