Doctors, including many of the 9,000 GP trainees, walked out at 8am Tuesday and joined around a hundred picket lines outside hospitals across England for the first of three planned strikes after contract negotiations collapsed last week.
The action followed 98% of junior doctors voting in favour of strikes at the end of last year.
Around 4,000 planned procedures are expected to be cancelled by trusts but emergency care will continue during this 24-hour walkout.
NHS England said everything possible was being done to ensure the provision of emergency care and minimise the effect on patients, but expressed regret that thousands of procedures would be rearranged and people would have to wait longer.
GP leaders told GPonline the action would not have a major impact on primary care services, despite fears of cancelled appointments or increased demand.
BMA GPC deputy chairman Dr Richard Vautrey said practices contracturally should not be relying on trainees.
GPs were, he added, 'overwhelmingly supportive' of the action junior doctors are taking. 'It shouldn’t have a major impact,' he said.
The BMA has said the latest contract proposal by the government dilutes safety provisions to guard against excessive workloads and would mean Saturday and some night working would be paid the same rate as weekday shifts.
A spokesperson for the union said: ‘Junior doctors now feel that they have no option. The biggest threat to patient care is the government’s insistence on removing safeguards which prevent junior doctors from being forced to work dangerously long hours without breaks, with patients facing the prospect of being treated by exhausted doctors.
‘The government is threatening to impose contracts in which junior doctors have no confidence and which represents the first step in a wholesale attack on all NHS staff at night and over weekends. We want a contract that is safe for patients, fair for juniors and good for the NHS.’
In a last minute appeal to junior doctors to call off the action prime minister David Cameron said the strike would cause "real difficulties for patients and potentially worse".
Mr Cameron said: "15 of the 16 points raised by the BMA have now been settled by NHS employers", reported Sky News, and dismissed as untrue claims that some doctors could see their pay cuts of 30% under new contracts.
GPs around the country will be visiting picket lines and showing support for junior doctors in other ways.
Writing for GPonline last week, Tower Hamlets chairwoman and Doctors in Unite/MPU GPC representative Dr Jackie Applebee said that if junior doctors lose their fight, pressure will increase on all NHS staff.
‘Put petitions and posters supporting junior doctors in your waiting rooms. Wear the BMA badges and stickers. Explain to patients that tired junior doctors make unsafe decisions. Visit picket lines to show your support’, she said.
NHS England’s director overseeing preparations for the strike Anne Rainsberry said: ‘We have tried and tested plans to deal with a range of disruptions including industrial action. As ever, the safety and care of patients is our top priority and the NHS has robust plans in place to ensure those who need emergency treatment will continue to receive it.
‘We have been working with hospitals and other NHS providers across the country to ensure we can continue to protect the safety of our patients and provide the urgent services they need. We will monitor the situation across the country to ensure these plans are in place, and are ready to respond to any significant increases in pressure in any region over the course of this dispute.’
A DH spokeswoman said: 'Thousands of patients will feel the impact of strike action - and it's clear that withdrawing emergency care is an extremely worrying prospect for the public. Polling shows that 8 out of 10 people think the NHS should provide the same standard of service to patients across the week — our central priority with these changes. Striking helps no-one and we remain hopeful that we can reach agreement to avoid further impact on services.'