The first strike by 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."
Liberal Democrat former deputy prime minister Nick Clegg told the BBC Radio 4 Today programme this morning: "No-one wants a strike and I understand the public concern about the effect of a strike, but equally my experience is that junior doctors, on the whole, are very hard-working men and women who also understand, as Jeremy Hunt has said, that people fall ill at weekends and need to be properly cared for.
"Something has gone wrong here and the Government needs to try to settle this as soon as possible."
NHS England said 1,425 inpatient operations and procedures were being cancelled as a result of the strike, along with 2,535 outpatient ones.
Some 654 cancellations - 192 inpatients and 462 day cases - are in London.
Junior doctors are set to provide emergency care only for 24 hours, starting from 8am this morning.
This will be followed by a 48-hour stoppage and the provision of emergency care only from 8am on January 26.
On February 10, there will be a full withdrawal of labour from 8am to 5pm.
The basis for the current round of negotiations is the Government's offer from early November, including an 11% rise in basic pay for junior doctors.
This is offset by plans to cut the number of hours on a weekend for which junior doctors can claim extra pay for unsocial hours.
Currently, 7pm to 7am Monday to Friday and the whole of Saturday and Sunday attract a premium rate of pay.
Under the Government's offer, junior doctors would receive time-and-a-half for any hours worked Monday to Sunday between 10pm and 7am, and time-and-a-third for any hours worked between 7pm and 10pm on Saturdays and 7am and 10pm on Sundays.
This is the first strike by junior doctors over pay and conditions since 1975, although they were involved in the 2012 walkout over pensions.
Jeremy Corbyn said the Government was to blame for the strike going ahead and called on ministers to apologise to junior doctors.
In a message on Facebook, the Labour leader said: "Everybody in Britain recognises and is grateful for the hard work and long hours put in by junior doctors. Their treatment by this Government has been nothing short of appalling, leading to the strike action in our NHS today.
"No NHS worker takes lightly the decision to strike, but the blame must be laid at the door of this Government for the way it has treated doctors and now seeks to smear them in the press.
"It is time for this Government to apologise to junior doctors and negotiate a fair deal that gets our NHS working again."