Hundreds of thousands of Armenians died while being forced out of Turkey
A Turkish prosecutor has opened an investigation against the authors of an online petition that apologises for the World War I massacres of Armenians.
It could lead to charges under the law against "insulting Turkishness".
Turkey's prime minister has criticised the petition, launched by more than 200 Turkish academics and newspaper columnists last month.
Hundreds of thousands of Armenians died at the hands of Ottoman Turks in 1915. Turkey denies that it was genocide.
It argues that those who died were just victims of the turmoil of World War I, in which many innocent Muslim Turks also died.
Discussion of the issue is taboo in Turkey and has led to prosecution in the past.
The intellectuals behind the petition say they want to challenge the official denial and provoke discussion in Turkish society about what happened.
The petition is entitled "I apologise".
Police said on Friday they had information that a man arrested this week was gathering intelligence and planning an attack against one of the organisers of the apology campaign.
Police said he had recently travelled to several cities and made it known that he was planning an action that would "shake Turkey", the BBC was told by a source who did not want to be named.
The man's uncle was reportedly among more than 30 people arrested on Wednesday in connection with an alleged ultra-nationalist coup plot.
Documents found in the raids led to the discovery of weapons and explosives in the forest outside Ankara, police say.
Some Turkish writers, who have promoted more open discussion of the Armenian issue, have been targeted by ultra-nationalist Turks.
The Turkish-Armenian writer Hrant Dink was killed last year for openly saying that the events of 1915 were genocide.
Previously he had been tried for "insulting Turkishness" for his comments on 1915.