The strike —taking place over two days — marks the first time that even emergency services are affected by industrial action.
The strike reflects the impasse between the government and the junior doctors, who are physicians with up to 10 years' experience, the Associated Press reported.
UK Health Secretary Jeremy Hunt has appealed to the medics not to drop emergency cover, but insisted that the government would not be "blackmailed" into scrapping an election pledge to bolster weekend services.
More than 125,000 appointments and operations have been cancelled and will need to be rearranged as a result, over the dispute between the government and the British Medical Association.