Ankara (AFP) - An Istanbul court on Thursday sentenced two prominent Turkish journalists to two years behind bars for illustrating their columns with a cartoon of the Prophet Mohammed originally published by French satirical weekly Charlie Hebdo.

The sentence handed to columnists Hikmet Cetinkaya and Ceyda Karan, both columnists with the opposition Cumhuriyet daily, intensified alarm over press freedoms in Turkey under strongman President Recep Tayyip Erdogan, who has made Islam the cornerstone of his politics.

"The two journalists were sentenced to two years each in jail," said their lawyer Bulent Utku. "We will appeal the ruling at the appeals court," Utku told AFP following a hearing at Istanbul's criminal court.

The pair, who went on trial in January last year, were acquitted of "insulting religious values" but convicted on charges of "inciting public hatred".

They were sentenced to three years in jail, which was reduced to two by the court on technical grounds.

- 'Erdogan's family among plaintiffs' -

The state-run Anatolia news agency said the case was brought by a total of 1,280 plaintiffs including Erdogan's daughters Esra and Sumeyye, his son Bilal and his son-in-law, Energy Minister Berat Albayrak.

The Erdogan family was represented by a lawyer in court, it added.

After the verdict, members of the public who had brought the complaint and were present in court shouted "Allahu Akbar", Cumhuriyet reported -- Arabic for 'God is greatest'.

On January 14, 2015, Cumhuriyet had published a four-page Charlie Hebdo pullout translated into Turkish marking the French satirical weekly's first issue since a deadly attack on its Paris offices by Islamist gunmen earlier that month.

The edition did not include the controversial front cover featuring the Prophet Mohammed, but a smaller version of the cartoon was included twice inside the newspaper to illustrate columns on the subject by Karan and Cetinkaya.

Most other media in Turkey had refrained from publishing the cover.

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When the edition was published, the daily received threats and Prime Minister Ahmet Davutoglu condemned the publication of cartoons of the Muslim prophet as an "open provocation".

Days before Cumhuriyet's special edition, Davutoglu had joined dozens of world leaders in a march through Paris, in memory of the 17 victims killed in three days of attacks at Charlie Hebdo's offices and elsewhere in the French capital.

- 'Shameful, unbearable'-

Cumhuriyet, which staunchly opposes the Islamic-rooted government of Erdogan, has been regularly targeted by prosecutions as concerns grow over freedom of speech in Turkey.

Its editor-in-chief Can Dundar and Ankara bureau chief Erdem Gul are currently on trial on charges of revealing state secrets and could face multiple life sentences if found guilty.

Writing on Twitter, Karan lashed out at the Turkish government, saying: "Let our two-year sentence be a gift for our liberal fascists #JeSuisCharlie."

In the controversial cartoon, the prophet sheds a tear and holds a sign with the viral slogan "Je suis Charlie" ("I am Charlie").

Most Muslims consider portraying the prophet in images to be blasphemous.

There has been growing concern about the numbers of journalists currently facing legal proceedings in Turkey, many on accusations of insulting Erdogan.

Trials for insulting Erdogan have multiplied since his election to the presidency in August 2014, with nearly 2,000 such cases currently open.

Reporters Without Borders slammed Thursday's verdict on its Twitter account as "shameful" and "unbearable".

In a show of solidarity, women's rights group Femen published on its Turkish Twitter page a picture of a topless activist holding a Charlie Hebdo cartoon with "Karan and Cetinkaya are not alone" written on her torso.

The case also comes as the government denies accusations it is seeking to wipe out the secular status of the overwhelmingly Muslim republic in a planned new constitution.

Europe is also looking to Ankara to implement a key deal to curb the flow of migrants seeking to reach the European Union from Turkish shores.

German Chancellor Angela Merkel, who spearheaded the agreement, has been accused of compromising on the EU's basic values on press freedom to win Turkey's cooperation.