As the Prague gunman relentlessly fired at his victims, an onlooker shouted “shoot here you f---er” in an attempt to distract him.

In a video circulating online, David Kozák can be seen on the balcony of the Charles University building dressed in black, shooting a rifle at those below him.

On the ground, a man can be heard screaming for the 24-year-old to shoot him instead.

After firing two shots in quick succession, the gunman disappears from view.

The onlooker then tells the police where to find Kozák through a walkie-talkie, according to the Daily Mail.

Kozák began his murderous rampage at the university at 2.59pm on Thursday.

Some students barricaded themselves in classrooms, while others escaped out of windows to hide on the building’s ledges.

Surrounded by police

Kozák killed 13 people at the school and injured a further 25 before shooting himself 21 minutes later after being surrounded by police.

The first victim to be named was Lenka Hlávková, the head of the Institute of Musicology at the university’s Faculty of Arts, and mother of two school-aged sons.

A representative of the city’s rescue service said those that needed surgery are in a stable condition. Among the injuries were gunshot wounds to limbs, chests and heads.

Earlier in the day, Kozák is also understood to have shot his father in their family home, and he has been linked to a double murder of a father and two-month-old daughter on Dec 15.

The killer legally owned eight guns, two of which were long-range. A police chief said they found “piles of ammunition” lining the corridors.

The massacre is the Czech Republic’s worst ever mass shooting, and Saturday has been declared a day of national mourning.