One in 10 patients have had their operations, outpatient appointments or treatments rescheduled due to the British Medical Association's (BMA) industrial action, figures suggest.
A quarter of GP surgeries are operating a reduced service due to BMA members participating in the day of action, the Department of Health said.
Doctors up and down the country are participating in their first day of action in 37 years in protest over the Government's controversial pension reforms.
The figures, which are early indications of the effect the action is having across England, show that there are varying levels of participation around the country.
In some places up to a quarter of GP surgeries are affected by the action. But in other areas "only a handful" of doctors are taking part.
Across London, 90% of hospitals are working normally, an NHS London spokesman said.
Hospitals in the capital have had to reschedule about 490 operations and reschedule 3,200 outpatient appointments.
There are 17% of GP practices providing a reduced service as a result of the action.
Of the 500 GPs in NHS Bedfordshire and Luton, 94 took part in the action, affecting 25% of practices in the county.
In Herefordshire, four out of 25 practices had GPs taking part in the action.
At Wye Valley NHS Trust, eight of 37 operations were rescheduled and 52 of 597 outpatient appointments were called off.
Hull and East Yorkshire Hospitals NHS Trust said 14 operations and 146 outpatient appointments were cancelled due to the action. The Trust usually performs 306 operations a day and sees around 461 outpatients.
NHS Hospitals in Nottingham saw seven of 140 operations cancelled.
In Dudley, all 53 GP practices remained open. Five practices confirmed they would be taking part in the action.
At the Medway NHS Foundation Trust, no operations were cancelled.
In Kent and Medway, there is at least one GP taking part in the action at 77 of the 273 practices in the area.
There were 331 outpatient appointments cancelled in hospitals in Barking, Havering and Redbridge. While 843 outpatient appointments are going ahead as planned.
In Leeds, very "few doctors" declared they would be taking part, a spokeswoman for Leeds Teaching Hospitals NHS Trust said.
Of 44 outpatient clinics due to be held at the Mid Staffordshire NHS Trust, only nine clinics have been affected.
Colchester General Hospital and Essex County Hospital have rescheduled 13 non-emergency appointments. This equates to about one eighth of the total number of non-emergency operations. Of 1,200 outpatient appointments, 160 were rebooked for another day.
University Hospitals of Leicester NHS Trust said that 27 operations had been cancelled. On a normal Thursday, 420 non-emergency surgeries would take place.
A spokeswoman said that 118 of 4,200 outpatient appointments were postponed.
The Royal Wolverhampton Hospitals NHS Trust said that just nine planned operations have been rescheduled and about 100 outpatient appointments were called off.
In the south west of England no GP surgeries were closed, with 671 of 1,821 having at least one member of staff taking part in the action. A spokesman for NHS South of England said 468 of 5,000 non-urgent operations were rebooked for another date.
Almost 3,500 outpatient appointments were affected compared with an average 47,000 outpatient attendances daily, the spokesman said.
In NHS Cumbria 38 GP surgeries out of 81 had someone taking part.
Health Secretary Andrew Lansley said: "In the run up to these strikes our objective has been to minimise disruption for patients. We asked doctors to recognise that their quarrel was not with patients but with the Government.
"I am pleased that a significant majority of doctors have done just that and maintained services for their patients.
"But let us not forget that the consequence of the BMA's decision to ask doctors to go on strike has been that thousands of patients who expected to have an operation or an appointment today have been inconvenienced or distressed by delay to their care.
"It is extremely regrettable if any patients have suffered unnecessarily. We will do everything we can to ensure that those patients get their treatment as soon as possible. I call on the BMA to commit to further co-operation to enable patients to have their rescheduled operations and appointments as soon as possible."
The BMA reluctantly announced the action last month after it accused ministers of pressing ahead with "totally unjustified" increases in pension contributions and a later retirement age for doctors, even though a deal on pensions was agreed four years ago.
All non-urgent work would be postponed, the BMA said, adding that although the action will be disruptive, doctors will ensure patient safety is protected.
Doctors will see anyone who is ill, or who believes they are ill, but will not do paperwork.
Accident and emergency departments and maternity services are running as normal, and tests for critical conditions such as cancer are still available.
But some non-emergency hospital clinics, outpatient appointments and planned surgery may have been be postponed.
GP practices remain open but some may have postponed routine appointments which can be rearranged for another day.
According to Mr Lansley, the planned action could see up to 30,000 operations cancelled, 58,000 diagnostic tests postponed and 200,000 outpatient appointments rescheduled.
Up to 1.25 million GP appointments will be pushed into the days and weeks following the action, he said.
The last time doctors took action was in 1975, when consultants suspended goodwill activities and worked to contract over a contractual dispute, and junior doctors worked to a 40-hour week because of dissatisfaction with the progress of contract negotiations.