The strike, which began on Monday, was called by two of the hospital doctors' unions and is scheduled to rumble on.
Unions have urged hospital doctors to down implements on Monday, and then each evening and each night throughout the week.
Around 80 percent of anesthesiologists are expected to take part, plus around 30 to 40 percent of specialist practitioners, the president of the union Avenir Hospitalier union told AFP.
Unions said that all planned surgeries would be postponed, though stressed that doctors will continue to care for patients and that help will be called in if needed.
Doctors are angry that they are being overworked to compensate for a serious staff shortage, which currently sees around 30 percent of hospital practitioner jobs sitting vacant.
As a result, doctors who shouldn't work for more than 48 hours each week end up working between 50 and 60.
Grégory, an anesthetist at the University Hospital of Rouen, told BFM TV that patients could be at risk from doctor fatigue.
"From time to time, we can arrive for a shift and we are already a bit tired," he said.
"It could have been a danger without me even noticing."
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This week's strike comes after France's health minister Marisol Touraine (pictured below) promised in 2015 to inject €250 million to improve working conditions. The funding has still not been spent.