By Can Sezer and Ece Toksabay

ISTANBUL (Reuters) - A world-renowned concert pianist was given a suspended jail sentence in Turkey on Monday for insulting religious values on Twitter, a case which has become a cause celebre for Turks alarmed about creeping Islamic conservatism.

Fazil Say, also a leading composer, went on trial in October for blasphemy - a crime that can carry an 18-month sentence - for a series of tweets including one citing a thousand-year-old poem.

That message, in April last year, retweeted a verse in which 11th-century Persian poet Omar Khayyam mocks pious hypocrisy. It is in the form of questions to believers: "You say rivers of wine flow in heaven, is heaven a tavern to you? You say two houris await each believer there, is heaven a brothel to you?"

In another tweet, Say poked fun at a muezzin, someone who makes the Muslim call to prayer. "The muezzin finished the evening prayers in 22 seconds ... Why are you in such hurry? A lover? A raki table?" he asked, referring to the aniseed-flavored spirit popular in Turkey.

The series of more than half a dozen tweets led prosecutors to accuse the 43-year old pianist of "explicitly insulting religious values".

An Istanbul court gave him a 10-month prison sentence but suspended it by five years on condition that he does not commit the same crime again in that period.

"Honestly we were not expecting this ruling, and all I can say is, both legally and for the country, it's a sad decision," Say's lawyer Meltem Akyol told Reuters.

Religious conservatives have become ever more assertive since Prime Minister Tayyip Erdogan's AK Party, which has roots in Islamist politics, swept to power a decade ago, arousing fears that Turkey's secular traditions are being eroded.

"Say did not repeat the words of a poet, but attacked religion and the holy values of religion, completely with his own words," said plaintiff Ali Emre Bukagili, a civil engineer and follower of a prominent Turkish creationist, who has brought a series of such cases against public figures.

DIVIDED OPINION

Say, who has performed with leading orchestras from Tokyo to Berlin, as well as the Israel Philharmonic and the New York Philharmonic, denied the charge.

"Fazil Say" became a top trending topic on Twitter immediately after the ruling was announced, with comments reflecting Turks' strong but divided opinions on the role of religion in public life.

"Scandalous and disgraceful," one tweet said of the ruling. "I wouldn't be surprised if a witch hunt for non-believers starts."

Another disagreed: "Finding religious values silly is one thing, provoking people through insults another. The court ruling is not wrong."

Erdogan's AK, its initials spelling out the Turkish word for purity, was elected in 2002 with a landslide. A decade since then of unprecedented prosperity is admired among Western allies keen to portray NATO member Turkey as a beacon of political stability in a troubled region.

But Erdogan's opponents accuse him of posing a threat to the modern, secular republic founded by Kemal Ataturk on the ruins of the Ottoman empire 90 years ago.

The courts have helped silence opposition and emasculate a military which was long the self-appointed guardian of Turkish secularism. It pressured an Islamist-led government from power in 1997 but has since been forced into retreat under AK rule.

Erdogan himself served time in prison in 1998, when military influence still held sway, for reciting a poem that a court ruled was an incitement to religious hatred.

The poem Erdogan had read contained the verses: "The mosques are our barracks, the domes our helmets, the minarets our bayonets and the faithful our soldiers."

(Editing by Nick Tattersall and Pravin Char)