South Korean police said they raided the offices of the Korean Medical Association on Friday, as the government contends with a doctors' strike that has led to chaos in hospitals.
Nearly 10,000 junior doctors -- about 80 percent of the trainee workforce -- walked off the job last week. They are protesting government plans to sharply increase medical school admissions to cope with shortages and an ageing society.
The government had set a February 29 deadline for medics to resume work or face potential legal consequences, including suspension of medical licenses and arrest.
Only 565 doctors resumed work by the deadline, according to figures released by the health ministry.
The mass work stoppage has taken a toll on hospitals, prompting the government to raise its public health alert to the highest level.
Around half of the surgeries scheduled at 15 major hospitals have been cancelled since last week, according to the health ministry.
Under South Korean law, doctors are restricted from striking, and earlier this week, the government requested police investigate people connected to the stoppage.
Police in Seoul confirmed a raid on the Korean Medical Association (KMA) on Friday.
The health ministry also posted on its website the back-to-work orders for 13 trainee doctors, leaving their licence numbers and parts of their names visible.
"We would like to inform you that refusing to comply with the order to commence work without justifiable reasons may result in disciplinary action and criminal prosecution," the order said.
"We express our gratitude for the wise decision of the trainee doctors who have returned to the patients' side," Health Minister Cho Kyoo-hong said in a statement released Friday.
South Korea's government is pushing to admit 2,000 more students to medical schools annually from next year to address what it calls one of the lowest doctor-to-population ratios among developed nations.
Doctors fear the reform will erode the quality of service and medical education, but proponents accuse medics of trying to safeguard their salaries and social status.
Junior doctors argue that the healthcare system's over-reliance on trainees is unreasonable and unfair.
Polling shows up to 75 percent of the public support the reforms.
President Yoon Suk Yeol, who has taken a hard line on the striking doctors, has seen his approval ratings tick up.
With legislative elections in April, and Yoon's party looking to win back a parliamentary majority, the government is unlikely to compromise quickly, analysts said.
The KMA has accused the government of using "intimidation tactics" to try to force doctors back to work, and said it was turning the country into a "totalitarian state".
The medical association will hold a rally in Seoul on Sunday, with local reports saying around 25,000 people are expected to join.
