Police in Beijing have arrested 12 scalpers who sell tickets for hospital visits after a scalping incident in a hospital trigged widespread public outcry.

The Beijing Municipal Public Security Bureau announced on Thursday that they had detained seven suspects at Guang'anmen Hospital following the report of a scalping incident there.

Four of them are still in police custody, and the bureau has set up a special working group to further investigate the incident, it said.

Police said they also arrested five suspected scalpers at Peking Union Medical College Hospital and Xuanwu Hospital, two renowned medical institutions in China, and they pledged to clamp down on such practices in Beijing.

A video clip that caught the public's attention on China's social media last week showed a woman at Guang'anmen Hospital in downtown Beijing on Jan 19 denouncing collusion between security staff at the hospital and scalpers.

She said she had waited in line only to be told the tickets were sold out, but scalpers in the line next to her later got tickets and offered to sell her one for 4,500 yuan ($690), far above the original 300 yuan.

Most hospitals require patients to reserve tickets to determine the order they receive nonemergency treatment from doctors. Patients can book online or by telephone, but the tickets through these channels usually have to be booked months in advance. Tickets for seeing an expert doctor can be only bought at the registration window in hospitals.

Patients who want to see a well-known doctor or want to have an immediate medical check have to buy registration tickets in person.

In some cases, scalpers put a chair and a glass bottle in front of the registration window to keep a place in line.

The National Health and Family Planning Commission, China's top medical authority, pledged to investigate and punish the scalping of hospital booking tickets after the video was made public.

Guang'anmen Hospital said it has no evidence that its staff colluded with the scalpers.

Last Friday, 32 scalpers were caught by police in the city's Haidian District at the Air Force General Hospital of the People's Liberation Army.