The junior doctor dispute is beginning to take its toll on the NHS in England, health service leaders are warning, as medics take part in their third strike.
Doctors walked out at 08:00 GMT on Wednesday in a row with the government over the imposition of a new contract.
NHS England's Dr Anne Rainsberry said the sustained nature of the action was making it difficult for hospitals.
Over 5,000 operations and treatments have been cancelled because of the current 48-hour stoppage.
It brings the total cancelled during the dispute to 19,000 with hospitals working hard to re-arrange the treatments, which cover routine operations such as knee and hip replacements. The NHS carries out about 30,000 procedures a day.
Thousands of check-ups, appointments and tests have been affected as well.
Dr Rainsberry, of 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."
Katherine Murphy, of the Patients Association, said she had "growing concern" about the dispute.
"Whatever the rights and wrongs of the arguments put forward by either side, the failure to resolve the differences by agreement is bad for doctors, bad for the taxpayer, but above all bad for patients and the NHS."
But despite the disruption polling released on the eve of the latest stoppage shows public support for doctors is holding firm.
The poll of 860 adults by Ipsos MORI for the BBC showed 65% supported doctors going on strike - almost the same proportion as backed them ahead of the walkout last month. Some 17% said they were against the strike, a drop from 22% last time.
The poll did show an increase in the proportion of people blaming both sides for the dispute. That now stands at 28%, up from 18%. However, the majority - 57% - still blame the government.
The strike comes as Health Secretary Jeremy Hunt is hosting an international patient safety meeting in London, which was planned before the strike was announced.
This week's walkout is the first of three 48-hour stoppages planned by the British Medical Association as they continue their fight against the government's plans to force through the changes to their pay and conditions. The next two are planned for April.
The union has also said it will be launching a legal challenge to oppose the imposition of the contract that was announced following last month's strike.
How far apart were the two sides?
- The BMA wanted everyone who worked on a Saturday to be paid at 50% above the basic rate
- Ministers only offered extra pay after 17:00 and at a lower rate of 30%
- But they have agreed to top up the pay by 30% for those who work regular Saturdays - defined as at least one in four
- Agreement was also not reached on on-call allowances, how limits on working hours are to be policed and days off between nightshifts
- The government offered a basic pay rise of 13.5%
- The BMA has said it was willing to accept between a 4% and 7% rise in basic pay to cover more generous weekend pay
On the eve of that walkout, government negotiators offered the BMA a last "take-it-or-leave-it" deal. It was rejected, prompting ministers to take the unprecedented step of forcing the country's 55,000 doctors to go on the new terms and conditions from the summer.
Mr Hunt said his hand was forced as the BMA was holding him to "ransom" and the changes were needed to help the NHS improve care at weekends. This is disputed by the BMA.
Dr Johann Malawana, the BMA's junior doctor leader, 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."