IT was like a scene from the Middle Ages.

The mob was gathered in a public square, angry and impossible to stop.

On a cleared patch of ground in the centre was a man, bound and blindfolded.

What ensued was a scene of such "utter cruelty" the photographer who bore witness to it is now desperately trying to erase the terrible scenes from his memory.

The photographer, who has asked to remain anonymous, was on assignment in Syria in the town of Keferghan, near Aleppo in the country's north.

As he recounted in an article published in Time magazine, he saw a "human being treated in a way that no human being should ever be treated". He said two and a half years of civil war in Syria had "degraded people's humanity".

While perpetrators of atrocities often record the acts on cameras or smartphones - and bloody photographs and videos abound on the internet - the images are often impossible to verify.

It was "very rare" for a professional photojournalist from outside Syria to gain "full and unfettered access to chronicle an atrocity as it unfolds".

This is what he saw: A crowd had gathered around a group of anti-Syrian Government rebels, possibly members of ISIS, an Al-Qaeda franchise operating in Syria and Iraq.

The mob had "no control over their feelings, their desires, their anger".

The victim was young, his hands tied behind his back.

"He was forced to his knees. The rebels around him read out his crimes from a sheet of paper."

The young man uttered something in Arabic in "an innocent and sad manner".

At that moment, the rebels "grasped his throat".

"The young man put up a struggle. Three or four rebels pinned him down," the photographer said.

The young man's attempts to protect himself from the onslaught were futile.

The rebels overpowered him and slit his throat, decapitating their victim.

They raised his head into the air in triumph and people "waved their guns and cheered".

It was the fourth public execution the photographer had witnessed on one day in the Aleppo region and had him on "the verge of throwing up".

"As a human being I would never have wished to see what I saw. But as a journalist ... I have a responsibility to share what I saw that day.

"I will close this chapter soon and try never to remember it."

See the terrible pictures at Time Magazine.

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