Police in Dalian, Liaoning province, have opened an investigation into a series of pictures posted online that showed two people partaking in "S&M" — sadistic and masochistic — activities, which involves participants using pain as sexual stimulus to gain pleasure, The Paper reported on Wednesday.

Netizens reported to police on Monday that two Sina Weibo users in Dalian uploaded inappropriate photos before later deleting them.

According to netizens who viewed the photos, the woman in the pictures had bruises all over her body and was chained up using metal locks. However, the woman did not appear concerned about her condition in her microblogs, but instead flirted with the man who appeared as her abuser in the photos.

Netizens said many of the photos were taken indoors, but that a few of them were probably taken in the Dalian Forest Zoo.

The case is under investigation and details cannot be disclosed yet, police said.

Gu Jingwen contributed to this story.