Despite last-ditch talks to prevent the damaging action, around 4,000 operations and procedures have been cancelled, with thousands more routine appointments also postponed.
Around 100 picket lines have been put in place for the strike.
Junior doctors are set to take strike action in a row with the Government over a new contract.
:: What is the dispute about?
The Government is intent on introducing a new contract for doctors working up to consultant level to replace one it says is "outdated".
Health Secretary Jeremy Hunt wants to introduce a "truly seven-day NHS". He plans to cut the number of hours on a weekend that junior doctors can claim extra pay.
Under the most recent proposals, doctors will receive an 11% rise in basic pay but extra pay for "unsocial" hours will be cut.
Currently, 7pm to 7am Monday to Friday and the whole of Saturday and Sunday attract a premium rate of pay.
Under the new plans, a higher rate would run from 10pm to 7am Monday to Friday, and from 7pm on Saturday evenings (a concession on the previous 10pm).
Mr Hunt argues that, under the new deal, just 1% of doctors would lose pay and those would be limited to doctors working too many hours already.
He said maximum working hours per week would fall from 91 to 72.
Johann Malawana, the British Medical Association's junior doctor committee chairman, has said the increase in basic pay is misleading due to the changes to pay for unsocial hours.
He said it devalues the vital work junior doctors do in the evenings and at weekends.
The BMA also has a range of other concerns, including on protecting doctors working in some specialties.
:: Haven't doctors threatened to strike before?
Three strikes were called off at the 11th hour on November 30 after the Government, the BMA and NHS Employers agreed to continue talks through the Advisory, Conciliation and Arbitration Service (Acas). There were three weeks of talks up to Christmas and then one day of negotiations, which failed to reach an agreement.
:: When are the strikes happening?
Junior doctors will provide emergency care only from 8am today.
This will be followed by two further spells of strike action, with a 48-hour stoppage and the provision of emergency care only from 8am on Tuesday January 26.
On Wednesday February 10, there will be a full withdrawal of labour from 8am to 5pm.