"We condemn the photographic exhibition at the European Parliament that displays extremist organization PKK's ringleader and PYD extremists, the Syrian affiliate of the PKK," according to a statement from the foreign ministry in reference to Abdullah Ocalan.

It said the European Parliament has acted as "an instrument" of propaganda for a extremist organization.

"Both ISIL and PKK are extremist organizations. Choosing one extremist organization over the other is disrespect to the memory of all the victims of extremism," it added. 

The approximately 30 photographs on display show PKK, PYD and YPG extremists in northern Syria.

In one image, crowds carry a photograph of Ocalan, who was convicted of extremism and treason in a Turkish court in 1999 and is serving a life sentence.

The photographs were taken by Austrian anthropologist Thomas Schmidinger and the Brussels exhibit is being hosted by Josef Weidenholzer, vice-president of the parliament’s Progressive Alliance of Socialists and Democrats.

The pictures, which purport to represent “important figures and historical events in the development of Rojava” -- a name given to regions in northern Syria -- will be shown until July 15, according to Weidenholzer’s website.

Turkey has repeatedly criticized such displays as serving to promote extremism.

In March, prior to a extrem attack in Brussels that killed 32 victims, the Belgian government allowed PKK supporters to pitch tents near the EU Council building on the eve of a Turkey-EU summit.