ISTANBUL (AFP) - Turkish police have over the last week detained at least 15 people for wearing a T-shirt with a slogan the authorities argue is a veiled message backing the alleged mastermind of last year's failed coup.

Police across the country have been detaining people wearing T-shirts with the word "Hero" in English in white capital letters against a black background, with the slogan underneath "Heroes are Immortal".

The authorities say the slogan is a veiled message of support for Fethullah Gulen, the US-based Islamic preacher blamed by the Turkish authorities for the July 15 failed coup aimed at ousting President Recep Tayyip Erdogan.

The controversy erupted on July 13 when a suspect tried over an alleged plot to assassinate Erdogan on the coup night was photographed going into court wearing the white "Hero" T-shirt.

This prompted outrage on social media, with users saying it was an insult to the 249 people killed at the hands of the coup plotters. Erdogan said that coup suspects should in future wear Guantanamo-style prison jumpsuits.

According to an AFP count based on reports on the Anadolu and Dogan news agency, at least 15 people have been detained over the last week for wearing the T-shirt.

Those detained have included a relative of one of the accused over the coup who attended a trial in Ankara wearing the "Hero" T-shirt.

A waiter working in a luxury hotel in the Mediterranean resort of Antalya was arrested after witnesses saw him wearing the T-shirt while going to work, Anadolu said.

In the latest arrests Saturday, a couple were arrested in Antalya and man in the western city of Canakkale for wearing the T-shirt.

The young couple in Antalya, who are both 22, were spotted wearing matching T-shirts on a motorbike and then chased down by police, the Hurriyet daily said.

Some of the T-shirt wearing suspects have been remanded in custody while others, like two students who wore the garment in the city of Eskisehir were released after saying it was bought by accident.

Those remanded in custody have been accused of making "propaganda for a terrorist group".

"Citizens are denouncing people who wear the "Hero" T-shirt and police are detaining them," wrote one user on Twitter.

"Our society has completely lost its mind."