Turkish journalist Ahmet Sik held 'over tweet'

Journalist Ahmet Sik (C) is a prominent campaigner for media freedom in Turkey

Police in Istanbul have detained a prominent investigative journalist, Ahmet Sik, in connection with his social media postings.

The arrest of Sik, who has been jailed previously, came shortly before writer Asli Erdogan and linguist Necmiye Alpay appeared in a Turkish court.

The court later released the pair pending their trial.

Many Turkish writers and journalists have been arrested since the July plot against President Recep Tayyip Erdogan.

The conservative ruling AK Party, which has Islamist roots, launched a massive purge of the police, judiciary, media, schools and universities after the abortive coup attempt by military officers.

• No cause for optimism in divided Turkey

• Turkey detains opposition journalists

Free speech campaigners Pen say the authorities have detained almost 150 writers and journalists.

Sik, 46, tweeted: "I am being detained. I will be taken to the prosecutor's office regarding a tweet."

He has been accused of spreading "terrorist propaganda", reports say.

He and journalist Nedim Sener were previously jailed in 2011-2012. The authorities banned Sik's book The Imam's Army, which examined the life and work of Fethullah Gulen, the US-based cleric blamed by the Turkish government for the July coup attempt.

A version of the book was however published in November 2011.

Kurdish links

Asli Erdogan, 49, and Necmiye Alpay, 70, are among nine suspects charged over their links to the pro-Kurdish newspaper Ozgur Gundem, which was shut down in August.

The authorities regard Ozgur Gundem as a mouthpiece for the banned Kurdistan Workers' Party (PKK), an armed group fighting for more Kurdish autonomy. Turkey and its Western allies regard the PKK as a terrorist organisation.

Opposition MPs rallied outside the Istanbul court in solidarity with Asli Erdogan and Necmiye Alpay

Asli Erdogan and Alpay - a prominent linguist and translator - both pleaded not guilty.

Asli Erdogan said she was on the paper's advisory board, but "it hasn't had a meeting in five years and hasn't taken one single decision".

One of her novels has been published in English, titled The City in Crimson Cloak.