This shocking video shows the moment a university student was struck to the ground and killed after she had protected two teenage girls who were being harassed.

A Serbian man, 18, is in custody over the November 15 attack, which left Tuğçe Albayrak, a student of German-Turkish descent, in a coma.

Her parents made the decision to switch off her life support machine on Friday - on her 23rd birthday - after doctors said she would never regain consciousness.

Her father has appealed for the girls she protected to help police with the investigation in to her murder.

Ali Albayrak said: 'Tuğçe will not come back, but you owe it to her to speak out.

'My daughter saved you, she did everything to ensure that nothing happened to you. She even perhaps died for you. So I appeal to you, please go to the police and make a statement.'

An 18-year-old suspect, identified only as Sanel M from the Sandzak region of south-west Serbia, is in police custody and is said to have admitted attacking Albayrak.

The video, published on the Bild website, shows the attacker getting out of his car and approaching Ms Albayrak and her friends.

A man repeatedly tries to stand between the attacker and Ms Albayrak, but the attacker manages to strike her on the head.

The video shows her falling to the ground and hitting her head, before the attacker flees the scene.

Ms Albayrak had intervened when she heard cries for help from the toilet of a McDonald's in the town of Offenbach, near Frankfurt, where the two girls, believed to be aged 13 and 16, were being harassed.

Later, one of the men returned and attacked her in the car park, striking her head with a stone or a bat.

Around 150 people attended a vigil in Berlin to honour her memory yesterday.

A petition calling for Ms Albayrak to be awarded the national order for merit posthumously has gathered more than 100,000 signatures.

German President Joachim Gauck has confirmed he will consider the award and described the student as a 'role model'.

He said: 'Like countless citizens, I am shocked and appalled by this terrible act. Tugce has earned gratitude and respect from us all.

'She will always remain a role model to us, our entire country mourns with you.

'Where other people looked the other way, Tugce showed exemplary courage and moral fortitude.'

Before Sunday's civil courage' vigil on Oranienplatz in Berlin, tributes were paid in Offenbach, with people holding signs which read 'Thank you, Tuğçe'.

Vigils have been held over the past two weeks at Justus Liebig University, where Ms Albayrak was a student, and at the clinic where she died.   

Police say an autopsy will be performed Monday to determine the exact cause of death. A funeral is due to be held this week.