A long-running battle over one man’s denial of the Armenian genocide returned to the European Court of Human Rights on Wednesday, drawing added attention by the presence of Amal Clooney.

The international human rights lawyer, who married Hollywood star George Clooney last year, was representing the Armenian government against a Turkish politician who denies a genocide took place 100 years ago.

Dogu Perincek, chairman of the Turkish Workers’ Party, was convicted eight years ago in Switzerland for describing the Armenian genocide of 1915 as an “international lie”.

He says that was an attack on his freedom of speech and the ECHR agreed with him in a December 2013 ruling.

The Swiss authorities have appealed the decision, and have the backing of Armenia, which says 1.5 million people were killed by Turkey’s Ottoman rulers.

Amal Clooney said the court’s 2013 decision “cast doubt on the reality of genocide that Armenian people suffered a century ago”.

Lawyers for Perincek and the Turkish government argue that the Armenian genocide is not a matter of “general consensus” like the Holocaust.

Turkey has always denied that the massacre of Armenians was a pre-planned attempt to wipe them out and says only 500,000 were killed.

Perincek “neither denied nor apologised for the massacres, nor did he incite hatred against the Armenians”, his lawyers argued, adding that he only denied a “genocidal intent” on the part of the Ottoman authorities who ruled Turkey at the time.

Turkey further argues that Perincek’s claims cannot possibly incite hatred since there is no widespread hatred towards the Armenians.

The controversy continued outside court where some 600 Turkish protesters had gathered, according to police, carrying Turkish flags and portraits of the country’s modern founder Mustafa Kemal Ataturk. The crowd cheered Perincek when he emerged from the court.

Around 20 Armenians stood on the other side of the road with one placard reading: “No to denial — Europe must react”.