Health secretary Jeremy Hunt has called on junior doctors to come back to the negotiating table and bring an end to "these very unnecessary strikes", as hospitals across England face disruption.
All but emergency treatments are facing disruption as junior doctors began industrial action following a dispute with the government over a new contract.
The walk-out went ahead at 8am and will last 24 hours, after a last-minute intervention by Prime Minister David Cameron failed.
Yesterday Cameron attempted to avert disruption to services by calling on junior doctors to cancel the strike, warning it would cause "real difficulties for patients and potentially worse".
However, a government source told the BBC that 38 per cent of junior doctors did go to work today.
So far hospitals have postponed some 4,000 routine treatments, with appointments and tests also affected, according to the BBC.
The British Medical Association, which represents close to 37,000 junior doctors, said it was left with no option but to strike after negotiations between the two sides broke down. Around 98 per cent of its members backed strike action in the November vote.
And the BMA isn't alone, with an Ipsos Mori poll showing the majority of the public support strike action as long as emergency care is provided.
However, health secretary Jeremy Hunt has said no junior doctor would be worse off under the new contract, with the offer of an 11 per cent rise in junior doctors' basic pay to compensate for the loss of overtime they currently earn.
Hunt has said the pay dispute was the most important aspect of the contract, and the government maintains 99 per cent of doctors will have their pay protected.
But Mark Porter, BMA council chair, has rejected the 99 per cent figure as "government propaganda".
Two more 24-hour strikes are planned for 26 January and 10 February. A third strike affecting all care will take place on 10 February between 8am and 5pm.