The strike, from 0700 GMT to 1600 GMT, is the fourth industrial action by doctors since the start of the year but the first to be extended to hospital emergency units.
Some 21,600 junior doctors took part in the first day of the strike yesterday -- 78 per cent of the number who had been due to work.
Prime Minister David Cameron condemned the strike called by the British Medical Association (BMA) union as the "wrong thing to do", but public support remains strong.
There are 53,000 junior doctors -- graduates with years of experience but who have yet to complete their training -- in England's National Health Service (NHS).
The taxpayer-funded NHS, which is administered differently in Wales, Scotland and Northern Ireland, was established in 1948 and is one of Britain's most respected institutions, providing largely free medical care.
While it has been shielded from austerity cuts to public services, experts warn it still faces increasing financial strain due to factors like rising treatment costs and an ageing population.
Several thousand junior doctors took part in a demonstration yesterday outside Cameron's Downing Street residence, where they were joined by Labour opposition leader Jeremy Corbyn.
