A spokeswoman for Sandwell Hospital in West Bromwich said it had declared a “level 4” incident and needed its doctors not to strike.
Dr Roger Stedman, medical director at Sandwell and West Birmingham Hospitals NHS Trust, said in a statement: “Over the last two days we have had very high numbers of patients come to hospital, and fewer than usual discharged.
‘Because of that we decided to require trainee doctors allocated to ward work to attend Sandwell during today’s strike.”
It comes as the first strike by junior doctors in 40 years is under way in a row over a new contract.
Around 100 picket lines have been put in place, with a large concentration in and around London.
A spokesman for the BMA said: “Junior doctors should continue with industrial action until NHS England has confirmed, and the BMA has agreed - via the agreed escalation process - that a major unpredictable incident is taking place for a specific trust.
“The BMA will notify members as soon as such an incident is in place.”
BMA chiefs previously claimed that hospitals would “move” the goalposts by declaring emergencies to force doctors off the picket lines.
In London, NHS singers gathered outside Great Ormond Street Hospital to lend their support to striking doctors.
Carrying banners, an 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 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.
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 BMA
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.
NHS England said 1,425 inpatient operations and procedures were being cancelled as a result of the strike, along with 2,535 outpatient ones.
Junior doctors are set to provide emergency care only for 24 hours, finishing at 8am tomorrow.
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.
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.”
Liberal Democrat former deputy prime minister Nick Clegg told the BBC Radio 4 Today programme: “Something has gone wrong here and the Government needs to try to settle this as soon as possible.”
Asked if further strikes would go ahead, Dr Kitty Mohan, from the BMA’s junior doctors committee, told the Today programme: “It is exceedingly difficult for junior doctors. This isn’t the reason why we go into medicine and have a medical career.
“Even today is breaking the heart of many junior doctors and therefore we really urge the Government to negotiate with the BMA.”