Turkish novelist Orhan Pamuk has remarked about the Armenian genocide. He stands accused of "insulting Turkishness" and could be jailed for three years if convicted.
A European Union official says the affair could have implications for Turkey's attempt to join the EU.
An excerpt from the BBC story:
Olli Rehn, who oversees Turkey's moves to join the EU, described the trial as a litmus test as to whether Turkey was committed to freedom of expression.
The writer has been charged with denigrating Turkish national identity.
He faces trial for remarks about Turkey's killing of Armenians during World War I and Kurds in the 1980s.
Ankara denies the deaths can be classed as a genocide and accuses Mr Pamuk of "insulting Turkishness".
The charges relate to a magazine interview earlier this year in which Orhan Pamuk said: "One million Armenians and 30,000 Kurds were killed in these lands and nobody but me dares talk about it."
He could face up to three years in jail if convicted.
European pressure
Mr Rehn said the "trial of a novelist who expressed a non-violent opinion casts a shadow" over negotiations for Turkey's entry into the EU.
He added that it presents an "opportunity to set a positive precedent for the numerous other cases of free speech that are awaiting trial".
"It is not Orhan Pamuk who will stand trial tomorrow, but Turkey," Mr Rehn said.