(AFP) - Turkey came under fire Thursday for halting a landmark conference questioning the official line on the mass killings of Armenians under the Ottoman Empire, as European Union diplomats warned that Ankara's democratic credentials had taken a serious blow.

Istanbul's prestigious Bogazici University, where the gathering was to open Wednesday, put off the event after Justice Minister Cemil Cicek accused the participants -- Turkish academics and intellectuals who dispute Ankara's version of the 1915-1917 massacres -- of "treason." Cicek condemned the initiative as "a stab in the back to the Turkish nation" and said the organizers deserved to be prosecuted.

The killings, one of the most controversial episodes in Ottoman history, is rarely discussed in schools and the aborted conference would have been the first by Turkish personalities to question the official stand on the events. Several countries have recognized the massacres as genocide -- a theory Turkey fiercely rejects -- and Brussels has urged Ankara to face its past and expand freedom of speech.

"The remarks of the justice minister are unacceptable. This is an authoritarian approach raising questions over Turkey's reform process," a diplomat from an EU country told AFP on the condition of anonymity.

"Now it is a real watershed. We expect government action to correct Cicek's remarks," he said. "It's up to the government to decide what to do. Doing nothing would be also a choice, but certainly not in favor of Turkey's EU membership prospects."

Another EU diplomat regretted the postponement of the conference because it "would have reflected the evolution taking place in Turkish society." The EU is looking forward for the conference to be rescheduled, he said, adding: "The Europeans will keep on insisting that civil society has a great role to play in Turkey."

The Turkish media too lashed out at the justice minister, saying his outburst cast a pall on freedom of expression in the country and played into the hands of a mounting Armenian campaign to have the massacres recognized internationally as genocide.

"Zero tolerance to freedom," the Radikal daily trumpeted on its front page, while Milliyet's headline declared: "Democracy takes a blow."

"What, really, is treason? To hold a conference in order to start a debate in Turkey on a Turkish problem debated almost everywhere in the world, or to brand as 'traitors' people who may think differently at a time when Turkey is waging a battle for democracy in the face of many obstacles?" wrote columnist Murat Celikkan in Radikal.

"Cemil Cicek should resign as justice minister and if does not, he should be forced to do so," he said.