German cops are on the hunt for a 19-year-old man they allege is a demented killer who knifed a boy to death on camera before posting the horrific act online.

Police say Marcel H., of Herne, filmed the alleged murder of an unrelated local nine-year-old boy before sharing it on the so-called dark web.

According to the report, Marcel H. entered a chat room following the alleged killing and held his bloody hands up to a webcam.

One viewer, who reportedly is familiar with the accused, alerted police in the aftermath of the disturbing killing, according to the Daily Mail.

“I have just seen a boy murdered on the Internet,” the witness told police.

Authorities attended a family home where the murder allegedly took place on Monday and discovered the body of a young boy in a basement.

A massive manhunt is underway in the surrounding area for the accused, who police say is a martial arts enthusiast.

Marcel H. fled Herne – 24 kilometres west of Dortmund – wearing camouflaged clothing, according to local news site Haz.de.