The first strike by UK junior doctors in 40 years is under way as thousands walk out in a row over a new contract.
Around 100 picket lines have been put in place, with a large concentration in and around London.
The NHS Choir gathered outside Great Ormond Street Hospital to lend its support to striking doctors.
Carrying banners bearing the lyrics of their song, A Bridge Over You, the Lewisham and Greenwich ensemble of doctors, nurses and NHS staff braved the cold to support doctors who formed the picket line shortly after 8am.
The words on the banners read: “The NHS needs saving and they’re not listening but we’ve got something to say. You can save us, don’t let them break us. We are your doctors, let’s keep it that way.
“The NHS should be yours, let’s keep it yours. Your lives are what we stand for. So let’s keep it yours.”
The British Medical Association’s junior doctors’ leader, Johann Malawana, said conditions for junior doctors need to change.
In a video posted to the BMA’s Twitter site, he said doctors have “even been unable to get leave for their own weddings despite months – and even up to a year – of notification in advance”, adding that the situation “cannot continue”.
There are more than 50,000 junior doctors in England – a position covering all doctors up to consultant level.
They represent a third of the medical workforce, and just over 37,000 are members of the British Medical Association (BMA), which called the strike.
Despite last-ditch talks to prevent today’s strike, around 4,000 operations and procedures have been cancelled, with thousands more routine appointments also postponed.
Patients have been told hospitals are under pressure and asked to attend A&E only if they have a genuine emergency.
Instead, patients are being asked to make the most of other NHS services, including GPs, walk-in centres, the 111 phone line and pharmacies.
The British Medical Association (BMA) is being backed in its action by other unions, including Unite, Unison and the RMT.
Members of other unions are expected to join picket lines in their lunch hour and if they are having days off.
Prime Minister David Cameron pleaded with doctors on Monday to call off the action.
He said: “This strike is not necessary, it will be damaging.”