More than 3,000 BMA members are expected to take part in four days of industrial action with thousands of appointments likely to be cancelled.
Industrial action started at 07:00 GMT on Monday and will last until 07:00 on Friday.
Dr Oba Babs-Osibodu and Dr Peter Fahey, co-chairs of BMA Cymru Wales’ junior doctors committee, said: “We don’t want to be in this position but again, faced with inaction, we are left with no choice.
“Junior doctors are starting their careers earning £13.65 an hour in Wales. Is that all they are worth? They are providing lifesaving care after training for years and are shouldering up to £100,000 of debt.
“It is no surprise that we are losing doctors as they search for better pay and conditions elsewhere. Losing our doctors at a time when waiting lists are at record highs will mean patients suffering more than they are already.”
Following her re-appointment by the new first minister last week, Health Secretary Eluned Morgan said: “While we wish to address pay restoration ambitions, our offer is at the limits of the finances available to us at present and reflects the position reached with the other health unions for this year.”
